412 TREADWELL. [Vol. XVII. 



of the fourth quartettes are all equal in size, whereas in nearly- 

 all annelids there is a large 2d and a large 4d. The entomeres 

 also are much larger in other annelids than in Podarke. 

 These differences are important and will be discussed later. 

 (See p. 441.) 



Let me repeat that up to the time when the small cell X1.2 

 appears, the polar furrow at the lower pole of the Qgg retains 

 its original direction. At the 64-cell stage, however, the 

 entomeres elongate inward to a greater or less extent, and 

 the shiftings of position thus caused among the cells may pro- 

 duce a change of direction in the polar furrow. (See PI. 

 XXXVIII, Fig. 27.) The condition shown in PI. XXXVIII, 

 Fig. 27, appears only rarely, but often enough to show that 

 the polar furrow can no longer be relied on for orientation. 



Since the regular geometrical progression is now lost (some 

 cells, e.g., trochoblasts, do not divide again, while others divide 

 repeatedly), the later history can best be treated by considering 

 each group by itself. 



First Group of Ectomeres . — At the 64-cell stage these are 

 arranged as follows : 



Trochoblasts 16 



Intermediate cells .... 8 



Cross cells 4 



Rosette cells 4 



The trochoblasts, as already stated, do not divide again, but 

 form the larval locomotor organs. They become elongated 

 as development proceeds, prominent nucleoli appear in their 

 nuclei, and they stain with great difficulty. They are very 

 much narrower than in Amphitrite, agreeing in this respect 

 very closely with Lepidonotus (No. 22). (See PI. XXXVIII, 

 Figs. 29, 30, etc., ap\ etc.) Cilia appear on these cells soon 

 after their last division, about four and a half to five hours after 

 fertilization, and the embryos at first rotate slowly and later 

 swim at the surface, being at first positively heliotropic. The 

 primary prototroch thus formed is at first divided into four 

 distinct areas, the openings between being later filled with cells 

 having a different origin. As long as the separate cells could 



