No. I.] GAR-PIKE AND STURGEON. 35 



Blastida. 



In this blastida of Acipenser, however, the relations between 

 yolk and blastomeres may best be understood by referring 

 them to the conditions of the Gar-pike. The periblast-Hke floor 

 of the segmentation cavity in the latter form has now become 

 reduced to a few irregular yolk-bearing cells, which in number 

 and position correspond closely with the merocytes of PI. II, 

 Fig. 28. A reduction in the amount of yolk material might 

 clearly account for this change. As in Lepidosteus the lower- 

 most cells of the blastoderm are derived from cells whose cyto- 

 plasm is connected with the yolk, and are budded off, as in PI. 

 II, Fig, 29, from central as well as from marginal regions. 

 The cells connected with the yolk are seen undergoing nuclear 

 divisions, and appear to represent in function the merocytes of 

 Lepidosteus. The irregular polygonal cells of the yolk hemi- 

 spheres are found to possess dividing nuclei close to their outer 

 surface, but are altogether indistinguishable in sections. Nat- 

 urally, however, the elaboration of the yolk material, although 

 occurring on every side, is most actively carried on in the 

 uppermost part in connection with the cell growth of the blas- 

 toderm. 



In a slightly later blastula the increased size of the segmen- 

 tation cavity and its more definite outline may be seen in sur- 

 face view, PI. Ill, Fig. 45, and in section, PI. IV, Fig. 60 (with 

 this cf. Ref. 29, PI. XV, Fig. 11). By study of the nuclear fig- 

 ures of the cells roofing the segmentation cavity in the preced- 

 ing stage, it is found that the present two-celled segmentation- 

 roof has been the result of horizontal cell cleavage. The 

 thickened sides of the cavity appear due to the normal increase 

 of marginal cells together with additions from the underlying 

 yolk cells. These in the floor of the segmentation cavity have 

 now produced a double layer of cells, whose upper elements 

 are in size, texture, and nuclear character not to be distin- 

 guished from those at the sides. The region, in other words, 

 of the yolk cells is retreating centrad. The writer notes that 

 his description of this stage of the blastula does not agree with 

 that of a corresponding stage of the sterlet given by Salensky. 



