Experimental Studies on Germinal Localization. 



211 



larvae seem to differ somewhat from those figured by Patten, 

 which show an extremely distinct apical plate. The later stages 

 are in the main similar to those described by Patten, and need 

 not here be considered. 



III. 



THE DEVELOPMENT OF ISOLATED BLASTOMERES. 



In general, as Crampton ('96) found for the 2- and 4-cell 

 stages of lUyanassa, the isolated blastomere, at whatever stage 

 it be separated' from its fellows, continues to segment essentially 

 in the same way as if forming a part of a whole embryo; but a 

 point on which I would lay stress is that there is a tendency for 

 all unequal divisions to be less unequal than in the normal devel- 

 opment, though this is by no means always the case, and the 

 isolated blastomere often divides exactly as in a whole embryo. 

 The partial character of the cleavage is also frequently masked 

 by shifting of the cells, and the partial embryos often close, some- 

 times at a very early period. Such shifting or closure appears, 

 however, to have no effect on the differentiation of the cells, as 

 is shown with especial clearness by the history of the trochoblasts. 

 Differentiation takes, in the main, the same course as if the cell 

 had remained united to its fellows, and gives rise to structures 

 that agree in a general way, and sometimes exactly with the parts 

 to which the cells would have given rise in a complete embryo. 

 For the sake of clearness I shall not follow the most logical or- 

 der, but will present first the cases that most completely sustain 

 the above statement — namely, the blastomeres of the first quartet. 

 It may be premised that all of the isolated blastomeres assume a 

 nearly or quite spherical form before division occurs, showing 

 no trace of flattening on one side; and they are indistinguishable 

 from one another except in size, and in the slightly greater trans- 

 parency of the micromeres. It is also necessary to bear in mind 

 that both in Dentalium and in Patella the eggs from different fe- 

 males vary very considerably in size, so that exactly correspond- 

 ing blastomeres from different eggs likewise present consider- 



