Experimental Studies on Germinal Localization. 29 



chal and post-trochal regions described above. Like the AB halves 

 and the j4-larvae, they often swim actively at the surface, rotating 

 in the same way as an entire larva; though the progressive move- 

 ment is almost always slower and less regular than that of the 

 smaller halves. These forms, however, seem to live no longer than 

 the less regular ones, and in spite of every precaution they become 

 more and more irregular and finally disintegrate in the same 

 aquaria containing the normally developing whole larvae. Those 

 that lived to the end of the second day invariably became mon- 

 strous in form and showed no resemblance to a normal larva. The 

 history of the AB halves or the smaller quarters in general very 

 closely resembles that of the lobeless larvae, the pre-trochal 

 region enlarging, becoming irregular, and finally disintegrating, 

 often while the embryo is still actively swimming by means of 

 the trochoblasts, which, as Fischel has observed in case of the 

 swimming cells of ctenophores, are most tenacious of life of all 

 the cells. 



The relative volumes of protoplasmic substance contained by 

 these various forms of larvae, may be determined either by meas- 

 uring the volumes of the blastomeres after isolation by means of 

 calcium-free sea-water, or by measuring the polar lobe and es- 

 timating the other volumes, the two methods giving fairly consist- 

 ent results. It should be remembered, howev^er, that both the 

 whole eggs and the relative size of the polar lobe (and hence of 

 the blastomeres) vary somewhat, both in the eggs produced by 

 a single female, and to some extent in those produced by dif- 

 ferent females. I observed one. lot of eggs, for instance, the 

 greater number of which produced lobes considerably smaller than 

 usual. Measurements of the lobe in typical average trefoils give 

 a value ranging from one-fifth to one-sixth that of an entire 

 egg. A typical case gave a volume of 0.18 for the lobe, from 

 which the other volumes are as follows : 



Entire embryo i-oo 



Embryo without polar lobe 0.82 



CD y2 embryo 0.59 



AB y2 embryo 0.41 



D y^ embryo 0.385 



, A, B or C 0.205 



