BY WILLIAM A. HASWELL. 99 



is, without much doubt, of the nature of a coagulum formed as a 

 result of the action of the fixing solutions on a fluid contained in 

 the segmentation cavity. 



Balfour (p. 53) remarks on the frequent presence of a 

 membrane-like structure between the blastoderm and the yolk, 

 readil}'' affected by staining agents, and sets it down as a layer of 

 coagulated albumen. Perenyi,* on the other hand, states that 

 in Torpedo marniorata, the yolk is enclosed in a fine structureless 

 membrane. 



The mass of fine-grained substance contains a small number of 

 nuclei. Its upper surface, forming the floor of the segmentation 

 cavity, is raised up here and there into a rounded mass containing 

 a nucleus. Continuous with the mass of parablast material which 

 lies below the blastoderm is a thinner layer extending out some 

 distance beyond the edge of the blastoderm. This is continuous 

 with the posterior edge of the latter, and at this point contains 

 several nuclei. Cell divisions at this stage seem to be going 

 on somewhat slowly, as the majority of nuclei are in the resting 

 stage. 



In Pristiurus, to judge from Balfour's account, the segmenta- 

 tion cavity makes its appearance only at a considerably later 

 stage. (Compare his figures 8 and 9, of Plate ii., and 1 of 

 Plate iii.) 



In the stages which immediately follow on that just described, 

 though the blastoderm (Figs. 2 and 3) does not at first increase in 

 size, the cells multiply by division so as to become much more 

 numerous and smaller. The massive blastoderm becomes much 

 thicker in front than behind. The segmentation cavity increases 

 in extent, and forms in the middle a comparatively wide space 

 covered over dorsally behind by a thin stratum formed by 

 material which is transitional between the posterior portion of the 

 blastoderm and the parablast. Cells soon cease to become 

 formed from the parablast of the floor of the cavity ; but a part 



• " Beitrage zur Embrj-ologie von Torpodo marmorata.'^ Zool. Anz. ix. 

 (1886). 



