8io 



DEVELOPMENT. 



[CH. LVIII. 



epiblast ; these cells multiply and the tube of epiblast gets much 

 thicker ; anteriorly it forms the brain and its cavity the cerebral 

 ventricles ; behind this it forms the spinal cord with its central 

 canal. The nerves grow out from brain and cord at a later date. 



d 



Fig. 624. Diagram of transverse section through an embryo before the closing-in of the 

 medullary groove, m, cells of epiblast lining the medullary groove which will form 

 the spinal cord; h, epiblast; d, hypoblast; ch, notochord ; , protovertebra ; sp, 

 mesoblast ; w, edge of dorsal ridge, folding over medullary groove. (Kolliker.) 



The same diagram shows that the mesoblast is collected into 

 large masses on each side of the neural canal ; these are called the 

 protovertebrce ; a rod of cells has been also pinched off from the 



Fig. 62$. Embryo of dog. The neural groove, A, is not yet closed, and at its upper or 

 cephalic end presents three dilatations, B, which correspond to the three divisions or 

 vesicles of the brain. At its lower extremity the groove presents a lancet-shaped dilata- 

 tion (sinus rhpmboidalis). c. Along the bottom of the groove is observed a faint 

 streak, which is the notochord. D, Dorsal ridges. (Bischoff.) 



hypoblast, and is seen in transverse section (ch) ; it is called the 

 notochord. 



Fig. 625 shows a surface view of the embryo at rather a later 

 stage. The union of the dorsal ridges takes place first about the 

 neck of the future embryo ; they soon after unite over the region 

 of the head, while the closing in of the groove progresses much 



