Localization of the N emertine Egg. 309 



and not vertical as in the whole egg. The fragments from three 

 of the eggs developed into cup-shaped half blastulae again, re- 

 sembling the similar embryos arising from isolated blastomeres 

 of the two-cell stage. There is, therefore, at this period a defi- 

 nite localization of cleavage factors. 



As regards the localization of the morphogenic factors no gen- 

 eral statement can be made. The two larvae obtained did not 

 show a sufficient differentiation to be of value. 



In the experiments on unsegmented eggs a study of the locali- 

 zation of the cleavage factors has been the main object in view, 

 the few and unsatisfactory isolated observations on larvae develop- 

 ing from the fragments being incidental and subsidiary to the 

 main point. In the following experiments, however, the study 

 of the localization of the morphogenic factors is definitely taken 

 up, the most extended series and the one yielding the most inter- 

 esting results being on the eight-cell stage. The experiments on 

 the localization of the cleavage factors are continued for the two- 

 cell and four-cell stages. 



VI. Two-cell stage. 



I. Experiments on the localization of the cleax-age factors in 

 isolated blastomeres. The blastomeres were isolated in twenty- 

 eight eggs of the two-cell stage. In the majority both blastomeres 

 segmented, a minority showing no cleavage of one of the parts. 

 In nearly every case the cleavage could be recognized as a partial 

 one corresponding with that of a lateral half of the whole egg. 

 At the four-cell (8/2) stage there is a wide cross furrow and the 

 cells are not in the same plane. In fact they appear very much 

 as if they had been removed from the whole eight-celled embryo 

 by a vertical cut (Figure 12 A, B). The different forms of cleav- 

 age described for isolated blastomeres of the two-cell stage of 

 C. lact-ens by Professor Wilson were found here also. Their 

 characteristics are especially prominent during the eight-cell 

 (16/2) and the sixteen-cell (32/2) stages (Figs. i2AtoI, 13A). 

 The most numerous are the cup-shaped embryos resembling a 

 geometrical half of a w^hole blastula of the corresponding age 

 (Figs. 12I, 12H). On the one hand the cups are replaced by 



