750 



DEVELOrMENT OF THE ^'EPLVO^S SYSTEM. 



enlarged, and presents a longitudinal median slit, analogous to the rliomboidal 

 sinus in bii-ds. 



The anterior fismirc of the cord is developed very earlj', and contains even at 

 fii-st a process of the pia mater. 



Fin 



<-/ 



icw'^' 







Fig. 550. — Transverse Sec- 

 tion OF Half of the 

 Spinal Cord of the Chick 

 OF Seven Days (from Fos- 

 ter and Balfour). Magnified. 



pciv, posterior, lew, lateral, 

 and acw, anterior •white 

 columns ; pc, jjosterior cornu 

 of grey matter with small 

 cells ; ac, anterior grey cornu 

 with large cells ; ep, epithelium 

 of the canal ; c, the upper 

 part now open and filled with 

 tissue in the posterior fissure ; 

 S2')c, the lower division of the 

 primitive medullary cavity, 

 which remains as the per- 

 manent canal ; «/, anterior 

 fissure left between the ]3ro- 

 jecting anterior columns ; ajc, 

 anterior grey commissure. 



: aye 



The cervical and lumhar 

 cnlarricmcnts, opposite the 

 attachments of the brachial 

 and crural nei-ves, appear at 

 the end of the third month : 

 in these situations the cen- 

 tral canal, at that time not 

 filled up, is some'ndiat larger than elsewhere (see figs. 556 and 5.58). 



At first the cord occupies the whole length of the vertebral canal, so that there 

 is no Cauda equina. In the fourth month the veitebraB begin to grow more 

 rapidly than the cord, so that the latter seems as it were to have been retracted 

 within the canal, and the elongation of the roots of the nerves which gives rise 

 to the Cauda equina is commenced. At the ninth month, the lower end of the 

 cord is opposite the third lumbar vertebra. (Kolliker, Ent'wickelungsgeschichte ; 

 Lockhart Clark in the Phil. Trans. 1862; Bidder und Kupfer, Untersuch. iib d. 

 Eiickenmark, Leipz., 1857. Foster and Balfom-, Elements of Embryology.) 



Till lately it was believed that the roots and ganglia of the spinal 

 nerves are at first distinct from the medullary substance of the cord, 

 ;and that they originate by differentiation of cells in the mesoblastic 

 f5ubstance of the protovertebral plate. But recent observations, to bo 

 more particularly referred to hereafter, have shown that they arise in 

 2)art at least in close connection with the spinal cord itself. 



THE BRAIN OR ENCEPHALON. 



1. — General phenomena of development as ascertained in birds and 

 ■onammals. — A reference has previously been made to the simple form in 

 which the brain at first presents itself in the anterior dilated portion 

 of the primitive medullary tube, and its partial division into the three 

 primary cerebral vesicles. This is placed within simple cranial walls 

 formed by the cephalic inflection of the blastoderm, without face or 



