Experimental Studies on Germinal Localization. 231 



C. DEVELOPMENT OF ISOLATED CELLS OF THE TWO- AND FOUR- 

 CELL STAGE. 



I have purposely left to the last an account of the development 

 of the half or quarter embryos, since this is in Patella in some 

 respects the least satisfactory part of the work. This is owing 

 especially to the great susceptibility of the Yi and M-larvse, which 

 frequently break up into smaller fragments, go to pieces, or be- 

 come quite abnormal during the cleavage process, so that very 

 few satisfactory larvae were obtained. In Dentalium the results 

 are much better, since the blastomeres can be easily separated 

 without the use of the calcium-free water; but even here my 

 fixed material has proved insufficient for a satisfactory analysis 

 of the internal phenomena. For these reasons the following ob- 

 servations remain somewhat fragmentary and must await a sup- 

 plementary study in these or other forms. 



I. Tlie partial cleavage in Dentalium. 



In my preceding paper I have described in a general way the 

 development of isolated halves and quarters in Dentalium, and 

 will here only add some details regarding their mode of cleavage, 

 which are hardly more than a confirmation of Crampton's results 

 on Ilyanassa. As in Patella, these earlier blastomeres, like the 

 later ones, become perfectly spherical after isolation before cleav- 

 age begins; their characteristic partial cleavage must therefore 

 be due to internal factors and not to their shape. 



The AB (anterior) half, which shows only an upper white 

 polar area (Fig. 83), segments equally into two, with no trace 

 of a polar lobe (Figs. 84-85) , and then forms by dexiotropic divi- 

 sions two slightly smaller micromeres of the first quartet, which 

 are composed entirely of white material (Fig. 86). The fol- 

 lowing division (Fig. 87) is like that occurring in half an egg, 

 the upper cells dividing slightly unequally to form below the 

 two primary trochoblasts ( la^ and ib") and above the two upper 

 cells (la^ and ib^). The lower cells in the meantime produce 

 the two cells of the second quartet (2a, 2b) in characteristic 



