GANOIDEI. 113 



medullary cord ; and there is no evidence of the epidermic layer 

 being at this or any subsequent period concerned in its form- 

 ation (vide chapter on Teleostei, p. 72). In the region of the 

 brain the medullary cord is so thick that it gives rise, as in 

 Teleostei, to a projection of the whole body of the embryo 

 towards the yolk. Posteriorly it is flatter. The mesoblast (Me) 

 in the trunk has the form of two plates, which thin out laterally. 

 The hypoblast (Jiy) is a single layer of cells, and is nowhere 

 folded in to form a closed alimentary canal. The hypoblast is 

 separated from the neural cord by the notochord (Ch], which 

 throughout the greater part of the embryo is a distinct structure. 

 In the region of the tail, the axial part of the hypoblast, the 

 notochord, and the neural cord fuse together, the fused part so 



we. 



FIG. 60. SECTION THROUGH AN EMBRYO OF LEPIDOSTEUS ON THE FIFTH DAY 



AFTER IMPREGNATION. 

 MC. medullary cord; Ep. epiblast; Me. mesoblast; hy. hypoblast; Ch. notochord. 



formed is the homologue of the neurenteric canal of other types. 

 Quite at the hinder end of the embryo the mesoblastic plates 

 cease to be separable from the axial structures between them. 



In a somewhat later stage the embryo is considerably more 

 elongated, embracing half the circumference of the ovum. The 

 brain is divided into three distinct vesicles. 



Anteriorly the neural cord has now become separated from 

 the epidermis. The whole of the thickened nervous layer of 

 the epiblast appears to remain united with the cerebro-spinal 

 cord, so that the latter organ is covered dorsally by the epider- 

 mic layer of the epiblast only. The nervous layer soon however 

 grows in again from the two sides. 



Where the neural cord is separated from the epidermis, it is 



15. TIL 8 



