DEVELOPMENT OF THE VERTEBRJE. 



dissimilar elements from its substance. In the first place, the outer half of each is 

 divided by a transverse fissure into two parts, of which the anterior, or that towards 

 the head, is afterwards converted into the root and ganglion of a spinal nerve ; 



Fig. 14, A. Transverse section through 

 the spinal part of the embryo represented in 

 fig. 13, at the place whei-e the spinal canal 

 is. still open, or is only a groove. 1, chorda 

 dorsalis ; 2, spinal canal or primitive groove ; 

 2 to 3, medullary plates, continuous with 4, 

 the corneous layer of the blastoderm ; 5, the 

 middle layer ; its outer part divided into two 

 laminae, the upper being the volunto-motory, 

 in which osseous and muscular plates after- 

 wards make their appearance, and the lower 

 being the involuuto-motory or intestino-fibrous 

 lamina ; 6, the epithelial, or lowest layer of 

 the blastoderm ; 7, the inner part of the 

 middle layer, in which the primitive vertebral 

 segments are developed. 



Fig. 14, B (adapted from Reraak and 

 Kolliker). Transverse section of the lumbar 

 part of an embryo more advanced, or towards 

 the fifth day, showing the spinal canal and 

 spinal marrow closed, the visceral plates and 



intestine formed, &c. 1, chorda dorsalis in its sheath ; 2, canal of the spinal marrow; 

 3, medullary substance of the spinal marrow ; 4, corneous layer ; 5, outer lamina of the 

 middle layer forming the visceral plates, and along with the corneous layer the amniou 

 4x5; 5', inner lamina of the middle layer or intestino-fibrous; the figure 5' is placed in 

 the peritoneal cavity ; immediately above it are seen the rudiments of the Wolffian bodies, 

 the aorta in the middle between them, and on each side the Wolffian duct; 6, the intestinal 

 cavity and epithelial layer, extending at 5', 6, into the blastoderm of the yolk-sac. 



and the posterior is the source of the transverse process of a vertebra and adjacent 

 portion of a rib. The inner part of the primordial vertebral mass passes inwards 

 in two modes: 1st, by its deepest part it passes above and below the chorda dorsalis, 

 and joining with the corresponding structures from the opposite side, surrounds 



Fig. 15 (from Remak and Kolliker). The cervi- 

 cal part of the primitive vertebral column and 

 adjacent parts of an embryo of the sixth day, 

 showing the division of the primitive vertebral seg- 

 ments into permanent vertebral arches, rudiments 

 of the spinal nerves, &c. 1, 1, chorda dorsalis in 

 its sheath, pointed at its upper end ; 2, points by 

 three lines to the original intervals of the primi- 

 tive vertebrae ; 3, in a similar manner indicates the 

 places of new division into permanent bodies of ver- 

 tebrae ; c indicates the body of the first cervical 

 vertebra; in this and the next the primitive divi- 

 sion has disappeared, as also in the two lowest 

 represented, viz., d and the one above ; in those 

 intermediate the line of division is shown : 4, points 

 in three places to the vertebral arches ; and o, similarly 

 to three commencing ganglia of the spinal nerves : the 

 dotted segments outside these parts are the muscular 

 plates. 



the chorda ; 2nd, its more superficial part completes the formation of a ring, or 

 a part of a cylinder, round the spinal cord. This may be considered as the 



