596 ON THE THEORY OF THE VERTEBRATE SKULL 



these are not real divisions ; they are appearances produced by the intersections of 

 the muscles which surround the notochord, and of the transverse vascular branches, 

 which accompany each pair of nerves when it leaves the medulla. [Dugfes thus 

 describes the somatomes.] 



In the second period cartilaginous processes, adherent to the notochord, appear 

 in pairs and enclose the medulla. There are as many of these processes as 

 vertebree will in future exist, and two crests even make their appearance, to form 

 the walls of the coccygeal canal. The apophyses are at first little tubercles ; they 

 then bifurcate ; one branch becomes the transverse process, the other the neura- 

 pophysis with its zygapophyses. In B. fusctis, A. obstetricans, pitnctatus, and 

 Hyla, where these vertebrae ossify, " two clouds " of ossific matter make their 

 appearance in each vertebra, " as distant from one another as they are from the 

 lateral masses or apophyses," and eventually unite above the notochord so as to. 

 form a quadrate ossific centre. This quadrate mass enlarges, but remains concave, 

 not only above, but also in front and behind, and especially below, where it forms, 

 a semi-canal or groove, in which the notochord is lodged. The groove is gradually 

 filled up, the notochord undergoing a contemporaneous atrophy, and becoming 

 eventually reduced to a mere ligament. The intervertebral masses are formed 

 altogether independently of the notochord. 



In Rana, on the other hand, the primitive centre of ossification of the body of 

 the vertebra is a ring completely enclosing the chorda ; in other respects the 

 development of the spinal column resembles that just described. 



2. Mijller (' Vergleichende Anat. d. Myxinoiden,' 1835) remarks, "that the 

 formation of the primitive elements of the skull (which are different from the 

 secondary osseous ones) takes place in the higher animals, constantly in the same 

 way, is much to be doubted, since variations of the fundamental plan obtain in 

 the vertebral column. In many Batrackia, as Cultripes provincialis, and Rana 

 paradoxa, the bodies of the vertebrae arise only out of the upper primitive vertebral 

 elements. I found, indeed, in the larva of Rana paradoxa, on the under part ot 

 the circumference of the chorda dorsalis, a cartilaginous band which was especially 

 well developed posteriorly, in front of the ossification of the coccygeal spine, and 

 was continued, thinner, along the under surface of the chorda, for half the length 

 of the future vertebral column. This cartilaginous band had no fellow, but on the 

 contrary, was thickest in the middle. In the caudal part of the chorda it 

 diminished until it gradually disappeared, so that the inferior arches surrounding 

 the caudal vessels were merely fibrous productions of the external sheath of the 

 chorda. But this inferior cartilaginous band on the chorda of the larva of Rana 

 paradoxa disappears in the greater part of the spinal column, and merely a part 

 of it ossifies to become the basilar part of the coccyx, which Dug^s was acquainted 

 with, as well as with the two vertebra of the coccyx above the chorda ; the basilar 

 bone is not a body of a vertebra, but coalesces subsequently with the inferior 

 circumference of the coccygeal vertebrse. In these frogs, the coccyx is the only 

 part which arises from both upper and lower vertebral elements ; all the other 

 vertebrae arise in Cultripes and Rana, merely from the upper primitive vertebral 

 elements, which in the course of ossification become divided into arches and 

 central portions. It is only the ossifications of the coccyx which, in these frogs, 

 completely enclose the chorda, since that part is eventually composed of two pairs 

 of vertebra, and a long basilar piece, whose sutures are retained even in the adult 

 R. paradoxa " (p. 1 30). 



3. Alytes. — With respect to the development of the vertebrae in Alylcs cbstet- 

 r/ftr«j-, Vogt (' Entwickelungsgeschichte der Geburtshelferkrote,' 1842) states that 

 cartilaginous rings appear in the sheath of the chorda, as rudiments of the 



