PISCES 191 



in viviparous Elasmobranchs, but they are well developed in 

 the skate and other oviparous forms. 



The horny case varies in shape according to the species. 

 In the skate and its allies it is a depressed rectangular case, 

 flat on one side and rounded on the other, and the corners 

 are drawn out into stiff, bent, tubular processes open at the 

 end. When the eggs are deposited these serve to anchor 

 the eggs to stones and zoophytes at the bottom of the sea, and 

 to produce currents of water of respiratory service. "When 

 development is completed after some months' incubation the 

 young skate emerges from an opening at one end. 



The segmentation of such a large ovum is necessarily con- 

 fined to the animal pole. The microscopical segmentation 

 nucleus divides, and the division is followed by a superficial 

 incision of the cytoplasm separating the two nuclei. The 

 yolk is altogether unaffected. Such a segmentation is called 

 meroblastic. As segmentation proceeds a mass of cells appears 

 at the animal pole, and this is called the blastoderm. Super- 

 ficially it has the appearance of a round patch of cells in which 

 the central elements are small and the peripheral next the 

 yolk rather larger. In section the patch proves to be several 

 layers of cells deep. A blastula is thus formed, and in some 

 Elasmobranchs the segmentation of the animal pole is followed 

 by late and partial indication of segmentation affecting to a 

 slight extent the yolk immediately outside the area. 



The conversion of such a blastula into a gastrula by an 

 invagination of the yolk is obviously impossible, and in spite 

 of the long history of invagination a return to delamination 

 is made. The cells of the blastoderm, or blastomeres, are 

 resolved into ectoderm and endoderm, the former external 

 and the latter internal next the yolk, and both are continuous 

 at the margin, which now becomes the mouth of the blasto- 

 pore. A segmentation cavity is also developed between them 

 in the process. 



The middle of the flattened ectoderm marks the anterior 

 end of the embryo, and the dorsal lip of the blastopore at once 

 becomes evident as the primitive posterior end ; and between 

 these two points, to begin with, the neural plate is formed, all 

 of which is destined to form the anterior part of the brain. 



