THE PROCESS OF CLEAVAGE. 65 



number, as is asserted by several authors (WALDEYER, 

 BALFOUR, etc.). This takes place by the constricting off of nuclei 

 and surrounding protoplasm, which go to enlarge the cellular disc. 

 We may, with WALDEYER, designate these as secondary cleavage-cells, 

 and regard the whole process as a kind of supplementary segmentation. 



By means of this a part of the voluminous yolk-material continues. 

 to be gradually individualised into cells. These annex themselves to 

 the border of the germ-disc, which with their aid increases in extent 

 and grows over a continually increasing territory of the unsegmented 

 yolk-sphere. In still later stages of development, long after the 

 cellular germ-disc has been differentiated into the germ-layers, the 

 supplementary segmentation continues to go on at the margin of the 

 disc in the neighbouring yolk-mass, and to furnish new cell-material. 

 Therefore the layer which encloses the yolk-nuclei forms an important 

 connecting link between the segmented germ and the unsegmented 

 nutritive yolk; I shall come back to this subject later. 



The appearance of merocytes and the supplementary cleavage 

 which proceeds from them are phenomena which are induced by the 

 vast accumulation of yolk-material, and which allow the latter to be 

 divided up into cells, even though the process is a slow one. 



The eggs of Selachians (KASTSCHENKO, RUCKERT) deviate a little 

 from the usual method of partial cleavage in meroblastic eggs, 

 and in a manner which recalls to a certain extent the processes 

 of superficial cleavage, which are to be treated of later. The 

 cleavage-nucleus, namely, is divided into two nuclei, these again 

 into four and even a greater number, without an accompanying 

 division of the germ-disc into a corresponding number of segments. 

 In this case, therefore, there arises at first a multinuclear proto- 

 plasmic mass, a plasmodium, in which the nuclei are distributed at 

 regular intervals. Subsequently furrows appear, generally in great 

 numbers and all at once, by means of which the germ-disc becomes 

 divided into cells from the centre to the periphery. Some of the 

 nuclei always remain in the periphery outside the territory of 

 cleavage, here undergo further division, migrate out of the germ- 

 disc into the surrounding nutritive yolk, and constitute the yolk- 

 nuclei or merocytes. These cause and maintain in the yolk for 

 a long time the process of supplementary cleavage. 



When we institute a comparison between partial and unequal 

 cleavage, for the descriptions of which we have made use of the eggs 

 of the Hen and the Frog, it is not difficult to derive the former 

 from the latter, and to find a cause for the origin of the former. 



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