ORGAN-FORMING SUBSTANCES IN EGGS OF ASCIDIANS. 2IQ 



of separation between these substances so that these various sub- 

 stances are ultimately segregated into -definite cells, but this per- 

 fectly definite type of localization arises without reference to cell 

 division and is not appreciably altered by subsequent divisions. 

 During the first cleavage the yellow crescent substance may be 

 seen to be undergoing complex vortical movements, but this does 

 not permanently change the form or position of the crescent. In 

 the unsegmented egg and in all the subsequent stages of the 

 cleavage the yellow crescent occupies its initial position on the 

 posterior side of the egg below the equator, irrespective of the 

 position of the cleavage planes. Likewise the gray crescent, the 

 ectoplasm and the endoplasm occupy the same positions in the 

 egg at the beginning of gastrulation as in the 2-cell stage ; in fact 

 so far as localization is concerned the condition at the close of 

 cleavage is the same as at its beginning. 



CVTOPLASMIC AND NUCLEAR ORGANIZATION. 



All of these different organ-forming substances are present, 

 and are shown in the photographs, as early as the close of the 

 first cleavage, some of them much earlier. In fact the clear ecto- 

 plasm, the gray endoplasm and the yellow mesoplasm are recog- 

 nizable in the ovarian egg. Here the mesoplasm forms a periph- 

 eral layer around the whole egg in which the "test cells" are 

 imbedded, the ectoplasm is contained within the large germinal 

 vesicle, while the endoplasm occupies the remainder of the cell. 

 Tracing these differentiations still further back it is found that at 

 least a portion of the mesoplasm comes from the sphere substance 

 (archoplasm), which is probably derived in part from the nucleus 

 of the last oogonic division (Conklin, 1902). The yolk also is 

 formed by the activity of the "yolk matrix" (Crampton, 1899) or 

 yolk nucleus which is probably derived from the sphere substance. 

 Portions of the ectoplasm, mesoplasm and endoplasm * are thus 



1 It may of course be objected that the yellow, the gray, and the clear substances 

 of the ovarian egg have not been proven to be differentiated for particular ends and 

 this I freely grant to be the case. Furthermore I do not see how this question could 

 be tested experimentally, especially in the case of the immature ovarian egg. The 

 fact that these substances are visibly different from one another in the oocyte and that 

 in all respects they resemble the ectoplasm, mesoplasm and endoplasm of the cleav- 

 age stages, to which they ultimately give rise, is the only reason for continuing to 

 call them by these names in this earlier stage. 



