436 MORGAN. [Vol. X. 



The sections show, moreover, a condition that had not been 

 appreciated in surface study. A certain amount of protoplasm 

 had collected beneath the cells derived from the blastomere 

 that had been left. This protoplasm must belong to the 

 blastomere removed. It is a well-established fact of teleostean 

 development that protoplasm from the surface of the yolk-sphere 

 continues to flow up into the blastoderm during the early 

 stages of cleavage. Apparently, this process continues in the 

 Q.g^ operated upon, although there are no cells on one side to 

 receive the protoplasm flowing up into this region. This I 

 believe to be the origin of the protoplasm that was found 

 accumulated beneath and to one side of the half-sized blastoderm. 



In my former contribution I stated that the protoplasm from 

 the yolk-sphere that continued to flow into the blastoderm 

 went, in all probability, entirely into the cells that remained. I 

 said : " Presumably the same process takes place in the Q.g% 

 operated upon, so that the half blastomere increases in size by 

 the protoplasm that it would receive had it remained in connec- 

 tion with the one removed ; but also must receive the additional 

 protoplasm that would normally have gone into the other re- 

 moved blastomere. Hence a blastoderm larger than half is 

 formed, and from this an embryo larger than half an embryo." 

 This assumption, my sections show, is not altogether true, 

 although indirectly the same result is brought about at a later 

 stage. 



My statement that "the size of the embryo is determined 

 by the amount of protoplasm present and not by the quantity 

 of nuclear matter" is, however, sufficiently demonstrated, I 

 believe, by the following experiment. " Often the first cleavage- 

 plane of the normal ^g^ divides the blastodisc into very unequal 

 parts (Fig. 25, A)} In some cases the larger blastomere has 

 been removed ; in others the smaller (Fig. 25, A). The result is 

 the same in either case, as a perfect embryo is formed ; but 

 the embryo is smaller when the smaller blastomere is left, and 

 larger when the larger blastomere remains." 



1 In Fig. 27 is drawn the four-cell stage of a normal egg, in which the first 

 division had been very unequal, more so than that of Fig. 2^, A. This egg was 

 isolated and a perfect embryo developed from it. 



