THE PROCESS OF CLEAVAGE. 



69 



composed of large yolk-cells, protrudes from this side far into the 

 cleavage-cavity, thus considerably diminishing it. 



The eggs with partial discoidal segmentation (fig. 38) are modified 

 most of all, and are therefore scarcely to be recognised as blastulae. 

 In consequence of the immense accumulation of yolk on the ventral 

 (vegetative) side, the cleavage-cavity (B) is extraordinarily constricted, 

 and is still preserved only as a narrow fissure filled with albuminous 

 fluid. Dorsally its wall consists of the small embryonic cells (kz) result- 

 ing from the process of cleavage, which are accumulated in several 

 superposed layers; at the surface they join each other closely, 

 deeper they lie more loosely associated. The floor of the cleavage- 

 cavity is formed of a yolk-mass, scattered through which are 

 to be found the 

 yolk-nuclei or 

 merocytes (die), 

 which likewise 

 result from the 

 cleavage-p r o c e s s. 

 It is to be seen 

 that they are espe- 

 cially numerous at 

 the place of tran- 

 sition from the 

 germ-disc to the 

 yolk-mass. 



dk 



Fig. 38. Median section through a germ-disc of Pristiurus in the 



blastula stage, after RUCKERT. 

 S, Cavity of the blastula ; kz, segmented germ ; dk, finely granular 



yolk with yolk-nuclei. 



This nucleated 



yolk-mass very evidently corresponds to the large vegetative cells 

 which constitute the floor of the cleavage-cavity in the case of the 

 Amphibian egg (fig. 37). 



In the case of superficial cleavage there is formed, strictly speaking, 

 no blastula, since the place where the segmentation-cavity should be 

 developed is filled with nutritive yolk. The latter either remains 

 unsegmented or is subsequently divided, as in the Insects, into in- 

 dividual yolk-cells. 



The investigation and right comprehension of the process of cleavage have 

 been attended with manifold difficulties. A voluminous literature has arisen 

 on this subject. We limit ourselves to pointing out the most important dis- 

 coveries and the chief questions which have been discussed. 



The first observations on the process of segmentation were made on the 

 Frog's egg. Aside from short statements by SWAMMEEDAM and ROSEL VON 



