GENERATION AND DEVELOPMENT. 



257 



The union of the medullary folds or laminae dorsales 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 more 



FIG. 411. Diagram of transverse section through an embryo before the closing-in of the medul- 

 lary groove, w, cells of epiblast lining the medullary groove which will form the spinal cord; ft, 

 epiblast; d, hypoblast; eft, notochord; w, proto vertebra; sp, mesoblast; w, edge of lamina dorsalis, 

 folding over medullary groove. (Kolliker.) 



slowly toward the hinder extremity of the embryo. The medullary groove 

 is by no means of uniform diameter throughout, but even before the dorsal 

 laminae have united over it, is seen to be dilated at the anterior extremity 

 and obscurely divided by constrictions into the three primary vesicles of 

 the brain. 



FIG. 412. Portion of the germinal membrane, with rudiments of the embryo; from the ovum of 

 a bitch. The primitive 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 dilatation (sinus rhomboidalis) c. The margins of the groove 

 consist of clear pellucid nerve-substance. Along the bottom of the groove is observed a faint streak, 

 which is probably the chorda dorsalis. D, Vertebral plates. (Bischoff.) 



The part from which the spinal cord is formed is of nearly uniform 

 calibre, while toward the posterior extremity is a lozenge- shaped dilata- 

 tion, which is the last part to close in (Fig. 412). 



Notochord. At the same time there appears in the middle line, im- 

 mediately beneath the floor of the medullary groove, a rod-shaped structure 

 formed by an aggregation of cells of the mesoblast; it soon becomes quite 

 distinct from the remainder of the mesoblasfc, and constitutes an axial cord 

 VOL IT. 17. 



