arise quite independently of the investing mass of the notochord. As regards the alisphenoids, they appear when they first take on a cartilaginous form to be distinct from the investing mass, in all animals above Batrachia: in young Tadpoles, however, as in young Sturgeons and in the embryos of Sharks and Rays, we find, in the place in which the alisphenoids are developed in other animals, an almost perpendicular thin plate of cartilage, which arises on each side immediately from a trabecula and the adjacent part of the investing mass. From this it seems to me very probable, that, at the time when these parts of the investing mass are still gelatinous, there grows up from them a very thin and easily overlooked plate-into which, in Frogs and most cartilaginous Fishes, the process of chondrification is continued uninterruptedly from the investing mass; while, in other animals, chondrification takes place only in separate spots and independently of those parts of the investing mass; the remainder of the plate being converted into a fibrous membrane, or into fibrous tissue of some sort.

§ 19. In the Snake, and probably in other Ophidia, the alisphenoids and orbitosphenoids grow upwards so far, that, at length, those of each side come into contact superiorly, so as to cover over and protect the brain. In most Vertebrata with an ossified skeleton, however, the growth of these bones is far more limited, in relation to the large cerebral mass, so that they leave an interspace superiorly over the brain. This interspace extends over and beyond the auditory capsules, as far as the exoccipitals; or even, when these parts do not come into immediate contact, between them to the end of the head.

But in order that the brain should be properly defended when this interspace exists, there arise within the soft parts which fill the interspace, and quite independently of other skeletal elements, special bony plates which, as they grow, become applied, partly to one another, partly to other cranial bones.

These are the two frontal* bones, the two parietals and the squama occipitis. The mode of origin of all these bony plates, which may be enumerated among the intercalary bones, depends upon the circumstance that those parts of the cerebral capsule which appear at the sides of the brain do not increase in size in the same ratio as the brain does. If the contents of the brain case become still greater, as in some cases of disease, even the addition of these intercalary

*No animal possesses more than two frontals. Cuvier, who assumed the existence of more than two, seems not to have interpreted these bones rightly.

bones is not sufficient to protect the brain, and then other, and sometimes very large bony plates are developed between and beside them, as may be observed in the hydrocephalic crania of Men and lower animals.

The squamose and mastoid parts of the temporal bone are such intercalary bones which are developed, not in all animals, but only in those whose brains acquire a considerable size in relation to the rest of the body.

§ 20. The investigations which have been made by Reichert and myself into the development of the skull of different animals, have yielded the following information respecting the development of the lower jaw, the upper jaw, the jugal, the pterygoid and the palatine. That part of the investing mass of the notochord in which the basi-" sphenoid is developed in many animals, sends out a 'ray,' or band, downwards on each side, which presents a remarkable similarity to a rib, not only in its mode of origin, but in its original position and form. But, very early, there grows out from near the upper end of the ray a long thin process, which passes off at an obtuse angle to it, and applies itself to the inferior wall of the future brain case. The whole structure now becomes separated from the part of the investing mass in which it originated, and becomes more or less displaced. The same operation takes place between the 'ray' and its processes.

The 'ray,' or outgrowth, now chondrifies, and except among the cartilaginous Fishes, there are formed round it many cartilaginous plates, which soon ossify, inclose it like a sheath, and in the end constitute one lateral half of the lower jaw. In the Mammalia, however, the 'ray' itself, after becoming greatly enlarged at its upper end, is metamorphosed into one of the auditory ossicles, viz. the malleus; whilst, in other Vertebrata it either remains throughout life cartilaginous and inclosed in the lower jaw, or (which however is not yet decided) only remains throughout the greater part of its length in this condition, its upper or smaller part becoming converted into the os quadratum. On the other hand, the process of the 'ray' elongating considerably, becomes developed into a pterygoid and a palatine bone -bones which in different Vertebrata come into more or less close connexion with other bones of the skull. Lastly, the upper maxilla and the malar bone are developed from a blastema which soon chondrifies, and is developed as a sort of coating upon the outer side of the process, just as the blastema out of which the lower jaw arises, originates around the 'ray.' The maxilla is, at first, remote from the pre

maxilla, but gradually the two grow towards one another and soon coalesce into one whole.

§ 21. The following results, among others, are deducible from the observations which have been detailed.

(1.) At the earliest period of foetal life the notochord extends backwards, as far as the end of the body; forwards, only to the interspace between the auditory capsules.

(2.) The gelatinous investing mass, which, at first, seems only to form a band to the right and to the left of the notochord, forms around it, in the further course of development, a sheath, which ends in a point posteriorly, but anteriorly sends out two processes which underlie the lateral parts of the skull, but very soon coalesce for a longer or shorter distance. Posteriorly, the sheath* projects but little beyond the notochord; but anteriorly for a considerable distance, as far as the infundibulum. It sends upwards two plates, which embrace the future central parts of the nervous system laterally, probably throughout their entire length.

(3.) The investing mass of the notochord is the material out of which the vertebral column and a great part of the skull, though not the whole skull, are developed.

(4.) The most essential part of a vertebra is its body. With the exception of a few cartilaginous fishes, the cartilaginous foundation of that body (the notochord having disappeared earlier or later), has the form of either a ring, or a half ring; or, as is the case among the Mammalia, constitute a solid mass, having the form of the segment of a cylinder. Subordinate parts of the vertebra are the vertebral arches and transverse processes, together with the ribs, which all, at the time they take on a cartilaginous character, appear as rays of the body-though sometimes they are not developed at all. Only in rare cases (Petromyzon) are vertebral arches developed without vertebral bodies, that part of the investing mass of the notochord which is, in other cases, applied to the formation of such bodies, acquiring only a membranous consistency.

(5.) From that part of the investing mass of the cephalic part of the notochord, which consists of the anterior part of the sheath of the notochord and its anterior paired processes, are developed the basioccipital, the basisphenoid and the ethmoid; so that the ethmoid is the most anterior of the parts of the skeleton which take their origin

* Perhaps with rare exceptions, as in Fistularia tabaccaria,

from the investing mass of the notochord. The basi-occipital is formed in that part of this mass which surrounds the cephalic part of the notochord, like a sheath; the basisphenoid in that part of it which lies between the paired processes (the trabecula) and the anterior end of the notochord; and the ethmoid (more particularly its body or pars perpendicularis), in the anterior coalesced part of those two processes. The body of the presphenoid, on the other hand, is formed below the processes in question, rarely between them.

(6.) The parts of the skull just mentioned, however, do not ossify in all vertebrata with an osseous skeleton; but one, or several, of them sometimes remain cartilaginous, and then grow, relatively, far less than the others, so that they seem to be pushed aside and suppressed by the neighbouring bones. This holds good especially of the basioccipital of the Batrachia, and of the basisphenoid of these animals and of osseous fishes.

(7.) The basi-occipital, or, at least, the substance out of which it will become developed, constitutes originally, like the body of a vertebra, a sheath round a part of the notochord; and the exoccipitals appear, whilst they chondrify, as outgrowths from the basi-occipital part, just as the arches of a vertebra, when this is normally developed, appear as outgrowths from its already chondrified body. For the rest, however, the normal development of the occipital bone is quite similar to that of a vertebra, and it therefore may with perfect justice be held to be a cephalic vertebra.* The squama occipitis, which occurs in many, but not in all Vertebrata, and which is not always placed between, but sometimes lies in front of the exoccipitals, presents no difficulty in the way of this interpretation; it is an accessory structure, a so-called intercalary bone, the presence of which depends upon the excessive development of the brain.

(8.) The two rings, on the other hand, which are formed by the two sphenoids, with the parietals and the frontals as their intercalary bones, are no longer constructed upon quite the same type as the vertebra. That the alisphenoids and orbitosphenoids, when they are already chondrified, do not appear to take the form of outgrowths of their centres, but are united with them by membrane, need not, perhaps, be taken very much into account; since, in the Lampreys, the arches of the vertebræ arise independently within the plates,

* The foramina condyloidea, which occur in the Exoccipitals of many Vertebrata, remind one of the holes in the vertebral arches of the sharks.

which the investing mass of the notochord has sent out to embrace the central parts of the nervous system. Still less weight can be attached to the circumstance, that not unusually, even when the sphenoidal centra are present, only one pair of the corresponding alæ appear; while, in other cases, two pair of alæ, and only one central part are present; since the caudal vertebræ of Mammalia usually exhibit no traces of arches, and the Lampreys have such arches without centra. On the other hand, it is an important circumstance that the basisphenoid, although it arises within the investing mass of the notochord, is not developed around this (though, so far as our present observations go, even the most posterior caudal vertebræ are so), but in front of it, in a process of the investing mass; and that the body of the presphenoid is no longer developed, even in a part of this mass (except in a few Mammalia) but arises quite independently of it. Hence the two sphenoids no longer agree perfectly with vertebræ in their development-the anterior diverging more widely from the vertebral type than the posterior.

(9.) Yet the two sphenoids, like the proper vertebræ, still embrace segments of the nervous tube (such as is formed by the brain and spinal marrow, at any rate in the early stages of development), and they constitute, as the vertebræ at first normally do, open rings, or rather, segments of rings, round that tube. The ethmoid, however, at no time surrounds a segment of the nervous tube in question; but, in a few animals, only imperfectly includes, by its hinder part, two anterior prolongations of that tube, whence the olfactory nerves arise. Its mode of development and its ultimate form likewise are of such a character that it no longer offers any special resemblance to a typically-formed vertebra. Nevertheless, considering that it arises from a part of the prolonged investing mass of the notochord, viz., from the anterior, early-coalescing, parts of the two trabeculæ, and that its body (the pars perpendicularis) presents even a certain resemblance to the last caudal vertebræ of many birds and osseous fishes, it well be considered to be a modified vertebra. We may look at it, in short, as representing only the body of a vertebra-as normally each caudal vertebra of a mammal does; and that from this, for the purpose of investing the olfactory apparatuses, which are developed at its sides, lamellar processes grow out, which are altogether peculiar to it. In any case, however, the ethmoid may be regarded as the anterior end of the vertebral column.

may

(10.) From what has been stated, it appears that the four dif