THE PROCESS oi < I. i:\VAOB. 



composed of largo yolk ivlls. protrndea tV<>m this >id- fir int., the 



cleavage-cavity, thus considerably liiniuishin^ it. 



The e_r^s with partial di>coidal .segmentation (fig. 38) are modified 

 most of all, and arc there! Iv to be recognised as blastuhe. 



In consequence of the immense accuinulat ion of \olk mi tin- v.-ntrul 

 t at ive) side, the cleavage-cavity (J' i-e\t ruordinarily <-.>n-t rioted, 

 and is still preserved only as a narrow limv tilled with albuminous 

 tluid. Dorsal ly 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- 



a yolk-mass, scattered through which are 



dk 



! *&"" 

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



blastula stage, after RCCKERT. 

 B, Cavity of the blastula ; L-:, segmented germ ; <llc, finely granular 



yolk with yolk-nuclei. 



cavity is formed of 

 to be found the 

 yolk-nuclei or 

 merocytes (dk), 

 which lik- 

 n-Milt from the 

 clea vage-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. 



This nucleated 



yolk-mass very evidently corresponds to the large vegetative cell- 

 \\liii-h 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, >t i ic' ly .-.peaking. 

 no blastula, since the place vln-iv the scpnrntat ion-cavity should be 

 developed is filled with nutritive yolk. The latter either ren 

 uni-egmented or is subsequently divided, as in th- . into in- 



dividual yolk-cells. 



HISTORY OF THE PROCESS 01 \om, 



The investigation and right compiviim>:.>n of the process of cleavage 

 been attended \\ith 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 ha\v l>e-n discussed. 



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

 Frog's egg. Aside from short statements by SWAMMERDAM and KOSLL vox 



