CHAP. X.] 



THE DEVELOPMENT OF THE GAT. 



321 



this groove is laid the foundation and commencement of the cerebro- 

 spinal axis, which, as we have seen, is the supreme and dominating 

 organ when that hody has attained maturity, and which is thus formed 

 from inflected epiblast. The groove becomes enlarged at its anterior 

 end, in the situation of the future brain (Fig. 145, C), while the 

 lateral margins of the whole groove grow up (as the dorsal plates or 



Fig. 144. DIAGRAM OF SECTION THROUGH THE GERM AREA OF A MAMMAL AT RIGHT ANGLES 



TO THE SURFACE. 



e. Epiblast. 

 ?'. Hypoblast. 



m. Mesoblast, or third layer. 

 7,-. Cavity of the vesicle. 



lamince dorsales) bend over and ultimately meet together above, 

 thus changing the groove into an elongated, hollow cylinder; the 

 central cavity of which is the precursor of the canalis centralis, of 

 the spinal cord, and of the third and fourth ventricles of the cerebro- 



Fig. 145. FIRST APPEARANCE OF THE EMBRYO, AS SHOWN BY THE MEDULLARY GROOVE. 



In A the germinal area is pyriform, and the 

 primitive groove occupies two-thirds of the 

 mi-row hinder end. 



In B the groove is elongated, and the middle 

 line and line closely surrounding it indicates 

 the rising up of the lamiruB dorsales or 



medullary plates on each side of it. 

 In C the embryo is seen somewhat expanded 

 towards each end. 

 at. Transparent area, 

 oo. Opaque area. 

 mp. Medullary groove. 



spinal axis as well as of the Her between them the enlargement at 

 the cephalic end assuming the form of three successive dilatations or 

 vesicles. Beneath the longitudinal groove a cellular rod becomes 

 developed, extending forwards as far as the hinder end of the front 

 cephalic vesicle and backwards to the end of the medullary groove. 

 This rod is the NOTOCHORD, or CHORDA DORSALIS, and occupies the 

 place of the future bodies of the vertebrae, the soft substance of the 

 intervertebral discs of the adult being its persistent remnant. 



