2<SO BASHFORD DEAN. 



its yolk-substance. There is of course still the possibility that 

 this total division is not the equivalent of total cleavage in other 

 vertebrates, for it may be due to the action of sperm nuclei a 

 possibility which finds some support in the subsequent history 

 of the extra-embryonic yolk masses (v. infra). This problem, 

 however, cannot be considered specifically at the present 

 time. 1 



Blastula. A blastula in an early stage is drawn in section 

 in Fig. 9. It shows a mass of blastomeres loosely arranged 

 upon a basis of germinal cytoplasm, traversed by extensive 

 intercellular spaces, the largest of which, .9. C., appears by 

 comparison with later stages to represent the cleavage cavity. 

 The topmost blastomeres are closely arranged and somewhat 

 epithelial in character ; the lowest are arising from the subjacent 

 germinal well. Particularly interesting is the fissuring (vacuolar) 

 of the subgerminal cytoplasm and of the neighboring yolk, for 

 many of these fissure-like vacuoles are seen to be directly con- 

 tinuous with intercellular spaces, and are, as has above been 

 noted, best interpreted as suppressed (or, better perhaps, retarded) 

 lines of cleavage. Asymmetry is already present in this stage, 

 for it will be seen that upon one side of the figure the blasto- 

 meres abut directly against the yolk, and that a more rapid 

 increase in cells is taking place nearer the opposite side of the 

 blastula. 



Gastnila. - - In Fig. 10, a section in which blastopore and 

 archenteron appear, is represented one of the most valuable stages 

 in my material, for I believe it furnishes us the key to the prob- 

 lem of the gastrulation of C/iinuera and also probably of sharks. 

 It shows in median section a dense mass of cells lying deep 

 within the germinal area, and closely apposed, save at one point, 

 (anteriormost) to the subgerminal cytoplasm. Near the opposite 

 (posterior) rim of the cell-mass, near the surface, is a small 

 cavity, Figs. 10 and 10 A, A, which communicates with the sur- 



1 Another problem which must here be neglected deals with the important relation 

 of yolk nuclei to mesenchyme. In this matter one may note, however, that Chimera 

 confirms Riickerfs position as to the conditions in sharks. The confirmation is 

 especially striking, since the additions to the mesenchyme budded out of the ger- 

 minal wall are here of gigantic size, and cannot be confounded with the neighbor- 

 ing mesoblast cells. 



