1007.] NATURAL SCIENCES OF PFIILADEr JMH A. 527 



becomes more marked with the separation of each quartet. The 

 <listal cells of this division, viz., la^ 16^^ Ic^ and IcP, correspond in their 

 method and time of origin to the "Trochoblasts" of annelids (Wilson, 

 92) or the "Turret cells" of mollusks (Conklin, 97). Their future 

 divisions correspond more closely perhaps to the trochoblasts. Since, 

 however, no structure directly homologous with the prototroch is 

 recognizable in the polyclad larva?, we cannot say that these cells are 

 homologous with those of annelids. 



The cells of the second quartet are slightly smaller than the original 

 cells of the first quartet. 



Fifth Cleavage — Sixteen to Thirty-two Cells. 



In the next cleavage each of the sixteen cells divides so that the 

 ideal thirty-two-cell stage is realized. Again the cells of the posterior 

 <iuadrant D divide slightly in advance of the others. The first cell to 

 divide is the large macromere 2D, which sends off in a dexiotropic 

 direction a small cell 3c/; 36 of the anterior quadrant is next separated 

 off. The other two cells of the third quartet may not come off until 

 after the cells of the first and second quartets have divided (fig. 14). 



The cells 2d and 26 begin to divide, shortly after the furrows for Sd 

 and 36 are formed. The divisions of the second quartet are nearly 

 meridional, but, as PI. XXXVII, figs. 16 and 19 show, the right cell is 

 slightly higher than the left, so that the division is dexiotropic. The 

 lower distal cells, 2a^-2d', are the larger (fig. 16). Before this division 

 has proceeded far the apical or stem cells of the first quartet divide in 

 a dexiotropic manner (fig. 12). The distal cells, la^-^-lrf*-, are some- 

 what smaller than the remaining apical cells (fig. 13). The former 

 lie in the angles between the latter, and the cells la^-ld^ are pushed out 

 until they lie opposite the ends of the apical cells. By this time the 

 other two cells of the third quartet, 3a and 3c, have usually been 

 constricted off (fig. 14). Next the remaining cells of the first quartet, 

 la^-lf/^ (trochoblasts), divide in a dexiotropic manner. In this case 

 tlie lower distal cells, la^-^-bi^-^, are somewhat smaller than their stem 

 cells (fig. 13). 



At this stage the cells of the third quartet, 3a-3d, are the smallest 

 cells present. The formation of this third quartet in polyclad eggs 

 was first observed by Lang, the older workers having overlooked it. 

 Lang calls this quartet the primitive mesoderm of the second order. 



The thirty-two cells are distributed as follows: 



