GANOIDEI. 



93 



concerned in its formation {vide chapter on Teleostei, p. 58). 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 (%) is a single 

 layer of cells, and is nowhere 

 folded in to form a closed ali- 

 mentary canal. The hypoblast 

 is separated from the neural cord 

 by the notochord (Gh), which 

 throughout the greater part of 

 the embryo is a distinct struc- 

 ture. 



In the region of the tail, the 

 axial part of the hypoblast, the 

 notochord, and the neural cord 

 fuse together, the fused part so 

 formed is the homologue of the 

 neurenteric canal of other types, 

 embryo the mesoblastic plates cease to be separable from the axial 

 structures between them. 



Fig. 59. Subface view of a Lepidos- 

 teds embryo on the fifth day after 

 impregnation. 



hr. dilated extremity of medullary plate 

 which forms the rudiment of the brain. 



Quite at the hinder end of the 



Fig. 60. Section through an embryo of Lepidosteus on the fifth day 

 after impregnation. 

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



In a somewhat later stage the embryo is considerably more elon- 

 gated, 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 epidermic layer of the epiblast 



