No. 2.] CLEAVAGE OF ARENICOLA CRISTATA. 79 



were ciliated, but I have never been able to observe any cilia 

 or any swimming motion in those raised at Naples. The larva 

 simply elongates and bursts the egg membrane and then twists 

 and squirms along the bottom and soon bores into the mud. 

 Vejdovsky's account of the cleavage is also extremely super- 

 ficial and careless. 



To sum up: as regards orientation, Arenicola and Ster- 

 naspis both agree with AnipJdtritc, but not with Nereis. As 

 regards the formation of the prototroch and the concrescence 

 of the somatic plate, Arenicola and Amphitrite agree, but both 

 differ from Nereis. Arenicola differs from Amphitrite in the 

 origin of the paratroch and in various details, and from Nereis 

 in the origin of the lips of the blastopore. 



The preceding account has dealt only with the cytogenetic 

 side of the question. The study of the cleavage has brought 

 out a number of facts of cytological interest as well. Suffice 

 it for the present, however, to say that cleavages occur which 

 appear to contradict all the so-called laws for direction of 

 spindle, etc. 



II. THE MOSAIC THEORY. 



Wilson ('93a) regards the cleavage of Nereis as "a visible 

 mosaic-work," and further asserts that "the principle of organ- 

 forming germ regions has here a real meaning and value, and 

 this would remain true even if hereafter it should be shown 

 that both of the first two blastomeres of Nereis, if isolated, 

 could produce a perfect embryo." 



However, if the production of a whole embryo from a half- 

 egg be possible, then the fate of the blastomeres is really after 

 all "a function of their position." Wilson ('93a) suggested 

 the possibility or probability of cellular interaction, thus depart- 

 ing from the true mosaic theory of Roux, of which the funda- 

 mental principle is self-differentiation. He still postulated, 

 however, true morphogenetic differentiation in the blastomeres. 

 More lately, as a result of Crampton's ('96a) experimental work 

 on Illyonassa, he ('96b) has reaffirmed his views. 



Lillie ('95) went so far as to say "the more precocious the 

 differentiation of the organs of the somatoblast, the greater the 



