The Development of Phascolosoma. 97 



(Fig-. 33). Since the seven blastomeres of the eight-cell stage which 

 divide first are very nearly equal in size but much smaller than ID, 

 which is the last to divide, we have an apparent confirmation of 

 Balfour's principle of the retardation of cleavage by yolk. This 

 idea, however, is so completely contradicted by later cleavage stages 

 in Phascolosoma, and by investigations upon other forms, that it 

 cannot be considered at present to have much weight, and, although 

 the posterior dorsal "macromere"', ID, is much larger than the other 

 blastomeres, its protoplasm cannot be said to contain a larger pro- 

 portion of yolk than they. The reverse, in fact, is probably true 

 as far as the first quartet of micromeres is concerned, for the latter 

 give rise later to trochoblasts, which show a marked tendency to 

 form yolk. 



The anterior hemisphere in the sixteen-cell stage consists of 

 the eight daughter cells of the first quartet of "micromeres" (Fig. 23, 

 28, 33), which are all of nearly equal size, and larger than any of 

 the cells of the posterior hemisphere, except those in the dorsal 

 quadrant, D. They very slightly exceed in size the ventral macro- 

 meres, 2A, 2B, 2C, and are considerably larger than the members 

 of the second quartet of micromeres that arise from them {2a— 2c), 

 which are literally micromeres. In the dorsal quadrant, D, 

 however, the remaining member of the second quartet, 2d, is a huge 

 cell, larger than its sister cell, the basal macromere, 2D. 



The anterior quartet of cells in the anterior hemisphere are to 

 give rise to the apical plate, the posterior quartet to the "primary" 

 blastomeres of the prototroch. 



Fifth Cleavage and Thirty-two-cell Stage. 



The regular rhythm of alternating cleavage continues, the 

 spindles which appear in the blastomeres of the sixteen-cell stage 

 being dexiotropic (Fig. 34). The first of the cells to divide are the 

 basal posterior blastomeres {2A — 2C) W'hich give ofi" the third quartet 

 of micromeres {Sa— 3c) (Fig. 24, 25), which are slightly smaller than 

 the blastomeres of the second quartet. The division of the cells of 

 the second quartet takes place immediately afterward. This division 

 is unequal, resulting in a minute anterior and a larger posterior 

 derivative. 



The division of the basal cell in the dorsal quadrant, 2D, is 

 retarded, as well as that of 2d, but it finally gives rise (Fig. 37, 38) 



Zool. Jabrb. XXIII. Abt. f. Auat. 7 



