96 John H. Gkrould, 



Thus we find that the first quartet of blastomeres , which sur-' 

 round the animal pole in the eight-cell stage, and which may be 

 called "micromeres" for the sake of uniformity of nomenclature, are 

 slightly larger in quadrants A, B, and C, than their sister cells at 

 the vegetative pole. 



This redistribution of protoplasm and j'olk in the four-cell stage 

 is the first movement towards the diiferentiation of the large yolk- 

 laden primary cells of the prototroch. which are to arise from the 

 first quartet of micromeres. It is an example of what Lankester 

 (1877) called precocious segregation, or the reflection of the later 

 stages of ontogeny upon the earlier, an idea which Lillie (1899) 

 has admirably brought out in his lecture on Adaptation in 

 Cleavage. 



A similar phenomenon has been noted by Coe (1899) in the 

 development of certain nemerteans [Micrura caeca, Cerehratulus leidyi 

 and C. lacteus), in which the cells of the first quartet are also 

 slightly larger than their sister cells, the "macromeres". The latter, 

 however, unlike the corresponding cells in Phascolosoma, are of the 

 same size in each of the four quadrants. A part of the descendants 

 of the first quartet in the Nemertea give rise to the apical plate, 

 and others become flattened and take part in the formation of the 

 extensive exumbrella of the pilidium. 



The division of A, B, and C, of the four-cell stage occurs nearly 

 simultaneously, and finally D, the largest cell, divides. This is true 

 of both species, except that, in Ph. goiildii, C. which is in that species 

 slightly larger than A or B, divides a little later than they, but 

 two or three minutes before D. Thus the principle of retardation! 

 of cleavage by yolk apparently holds true in the early cleavage of) 

 Phascolosoma. 



Fourth Cleavage and Sixteen -cell Stage. 



The spindles formed in the blastomeres of the eight-cell stage 

 are laeotropic (Fig. 22, 32). Division of both cells in each of the 

 three quadrants A, B, and C is nearly simultaneous, and there is 

 no appreciable regularity in the succession among the difl'erent 

 quadrants. In the dorsal quadrant, D, however. Id is sometimes the 

 first blastomere of all the eight to divide, whereas ID is regularly 

 the last. Spindles of the following cleavage are already formed in 

 most of the cells of the next generation before ID has divided; 

 thus a fifteen-cell stage arises in both Ph. goiddii and Ph. vidgarel 



