94 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



" pass into the yolk hemisphere, where it transforms yolk material into 

 protoplasm ; the second merocyte, formed partly in this way and partly 

 from previously existing protoplasm, issues as the second blastomere, 

 while the first becomes simultaneously cut off from the yolk . . . the 

 nucleus of the third merocyte is derived from that of the second ; the 

 latter becomes spindle-shaped, and gives off a nucleus, which, accom- 

 panied by little or by no appreciable quantity of protoplasm, passes into 

 the yolk. . . . The third merocyte, in similar manner, while emerging 

 as a blastomere, divides and gives off a nucleus to the yolk, which in a 

 similar manner gives rise to new merocytes and blastomeres." 



It is evident, as indeed Groom distinctly states in another place, that 

 he regards the yolk as non-nucleated and receiving nuclei from the suc- 

 cessively formed blastomeres. In the discussion of the first cleavage I 

 have pointed out that a nucleus from "the first blastomere" (the cell 

 ab z in this paper) does not pass into the yolk-cell just before the separa- 

 tion of the two cells. This also applies to all succeeding cleavages. The 

 yolk-cell does not derive its nucleus from successively formed "proto- 

 plasmic" cells ("blastomeres") such a description is inaccurate and 

 misleading. In no case can either " blastomere " or the yolk-cell be 

 said to derive its nucleus from the other, for the micromeres are merely 

 the result of ordinary unequal division, which differs from the division 

 of cell er6 2 in the inequality of the products, but not in the method by 

 which it is brought about. 



The term "merocyte" conveys the idea that the protoplasm is more 

 or less sharply distinct from the yolk, as in the case of eggs which un- 

 dergo superficial cleavage. This is evidently the idea intended to be 

 expressed in the above quotations from Groom. Neither living eggs 

 nor stained sections support such an interpretation. A considerable 

 part of the yolk-cell cd 2 is protoplasmic, the yolk and protoplasm being 

 so mingled that there is no justification for the use of the term " mero- 

 cyte." I cannot agree with Groom's statement that throughout the 

 main portion of its mass the yolk-cell contains little protoplasm. Pro- 

 toplasmic processes extend even among the oil droplets which lie near 

 the periphery at the vegetative pole of the egg (Fig. 27). I cannot 

 confirm the statement (p. 198) that there is little protoplasm left in 

 the yolk-cell immediately after the separation of a new blastomere, and 

 that the nucleus rapidly transforms yolk into protoplasm to form the 

 new blastomere. The amount of yolk is not very much diminished 

 before the sixth cleavage. This is in accord with the facts known 

 in the case of the development of other animals, for rapid transforma- 



