llli: PROCESS OF CLEAVAGE. 65 



im,,i? t -t', M \B MMTted by several author^ ( \V.\i. I.KVER, R0CK 

 BALFOUB, etc.), This take, plaeo by the coiiMi-i. ting oil 1 ot' nuclei 

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

 We niav, \vitli \\"AI.IU:\ i i: . d. ; j-j.ai. i In -so as secondary cleavage-ce\\s t 

 and regard the whole process as a kind of Kii/i/>tt.'in> />/"/,/ >-./, 



I'.v in- ans of this a part of the voluminous yolk material continues 

 to be gradually individualised into cells. These annex themselves to 

 i ho border of the germ-dis/, which with their aid increases in -. 

 and grows over a continually increasing territory of tin- un.-egmentrd 

 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 

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

 fore the layer which encloses the yolk-nuclei form.* an /////<////// 

 connecting link between the segmented germ and the unsegw 

 a nt, -it ic>> 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 

 accumulation of yolk-material, and wi.ich allow the latter to be 

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



Th Selachians (KASTSCHENKO, RL Y CKERT) deviate a little 



t'lom the usual method of partial cleavage in ineroblastic 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 segment- 

 In this ca>e. therefore, there arises at first a in ulti nuclear proto- 

 plasinic ma-s. aplasmodium, in which the nuclei are distributed at 

 regular intervals. Subsequently furrows appeal-, generally in i:re;t 

 numbers and all at once, by means of whicn the i:erm-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 di\i>ioii. migrate out of the {_' 

 disc into the >urroundinu r nutritive yolk, and constitute the j 

 nut-lei or meiocyto. These cause and maintain in the yolk for 

 a long time the process of supplementary cleavage. 



When we institute a comparison letween partial and unequal 

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

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

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



5 



