296 MEAD. [Vol. XIII. 



tions. The alternating oblique direction of the division of these 

 cells in ChcBtopterus compared with the strictly bilateral division 

 in all the other forms makes it doubly difficult to explain either 

 case on mechanical principles. 



I have already given my reasons for believing that Wilson is 

 mistaken in asserting that the bilateral period in Nereis begins 

 with the 38-cell stage. It certainly begins only after the 64-cell 

 stage in the forms I have studied. I disagree further with 

 Wilson ^ when he says that the cause of the introduction of 

 bilateral cleavage is the reduction of the " posterior macromere " 

 to the size of its fellow. Bilateral cleavage occurs in LepidonoUis 

 and in Umbrella where there can be no such "reduction," 

 because the macromeres are equal from the first. Moreover, in 

 Chcetopterus, where the posterior macromere is larger, certain 

 bilateral divisions which are always found in the other forms 

 do not occur. 



I do not understand how Kofoid could have inferred 

 from Wilson's or Heymons's figures or text that the rule of 

 alternating (spiral) oblique cleavage holds good throughout the 

 cleavage of Nereis up to the " 58-cell stage," and of Umbrella 

 up to the 9 1 -cell stage. The 58-cell stage in Nereis is brought 

 about by the obviously bilateral division of some of the cross 

 cells and of the mesoderm cells, while the 91 -cell stage in Um- 

 brella is attained only by the bilateral division of the mesoblast. 



Tlie rate of cleavage. — The absolute rate of cleavage varies 

 with the temperature of the water and among different species, 

 and cannot be predicted from the size or physical appearance of 

 the egg (cf. AmpJiitrite and Unio). In all polychaetes which I 

 have studied, the division of the oocytes of the first order, i.e., 

 the formation of the first polar globule, will not take place 

 until the sperm has entered the egg. Just how the entrance 

 of the sperm stimulates the cell to karyokinetic activity is unex- 

 plained, but the phenomenon is suggestive in that it shows 

 that, in one case at least, it is not the mass of the egg nor the 

 presence of more or of less yolk that determines the time of cell 

 division, but a stimulus of some kind, analogous, perhaps, to 

 that which starts into activity the motor apparatus of pigment 

 cells, leucocytes, or muscle cells. 



