36 DEAN. [Vol. XI. 



The latter author has figured an early blastula whose segmen- 

 tation cavity has a roof three cells in thickness, the sides one 

 cell in thickness, and a floor lacking in differentiated cells. 

 Salensky, in addition, figures the yolk mass totally traversed by 

 one cleavage plane and partially by a second. 



In a slightly later stage the walls of the segmentation cavity 

 have greatly thickened. The roof and sides of the cavity are 

 now of 8-9 cells in thickness and its floor is of 4-5 tiers of cells 

 which preserve their yolk-cell characters. Differentiated cells 

 in the lower hemisphere of the Q.gg are not to be distinguished. 

 Pigment is plentifully scattered in all cell layers of the roof of 

 the segmentation cavity. 



A later blastula has been figured in surface view in PI. Ill, 

 Fig. 46, and in vertical section in PI. IV, Fig. 61. It is com- 

 parable to the stage which Salensky has figured in PI. XVI, 

 Fig. 12, but differs from the latter notably. Thus its yolk 

 mass is not as yet divided into cells, the segmentation cavity 

 is larger, more definite in outline, its roof thinner at the animal 

 pole, its floor, slightly concave, composed of definite and uni- 

 form cellular elements. The stage figured immediately pre- 

 cedes gastrulation, and it is doubtless on this account that a 

 marked asymmetry maintains in the roof of the segmentation 

 cavity : below the thicker side will occur the dorsal lip of the 

 blastopore : outwardly this stage is of finely finished appear- 

 ance, in contrast with that of Fig. 43 ; continued division has 

 rendered the cells of the animal pole so minute that this region 

 appears waxy ; the marginal pigmented zone has in addition 

 lost its well marked appearance, and the cell outlines of the 

 lower hemisphere are distinguishable but faintly. The pigment 

 which now appears concentrated at the animal pole is in the 

 region where the roof of the segmentation cavity is thinnest 

 {cf. PI. IV, Fig. 61). 



Gastrula. 



Gastrulation begins at a stage represented in PI. Ill, Fig. 47, 

 and in vertical section in PI. IV, Fig. 62. Outwardly this 

 stage is closely similar to that last figured ; it possesses the 

 dark pigmentation at the animal pole, the white zone of minute 



