Experimental Studies on Germinal Localization. 63 



larvae are sometimes nearly normally formed except for the false 

 proportions of the post-trochal and pre-trochal regions. Their In- 

 variable subsequent degeneration Into Irregular and monstrous 

 forms Is not Improbably due to the abnormal mechanical condi- 

 tions created by their mode of development. It seems possible, 

 however, that if these larvae could sustain themselves sufficiently 

 long they might in some cases succeed in attaining a normal con- 

 dition. They die before attaining this end; and hence succeed no 

 better than the AB halves In the "attempt" to produce a perfect 

 embryo. 



One cause of the difference between the isolated blastomeres of 

 the nemertlne or sea-urchin and the mollusk thus doubtless lies 

 in a difference In the segregation-pattern such that In the former 

 the specific materials are symmetrically divided between the first 

 two blastomeres, while in Dentalium such is not the case. In the 

 former, accordingly, the earlier cleavages are purely quantitative, 

 but in the latter are qualitative as far as the cytoplasm is con- 

 cerned, and to this extent produce from the first cleavage onward 

 a mosaic-work in entire accordance with Roux's general concep- 

 tion, as I long since indicated in the case of Nereis ('94). But 

 beyond this the results especially of Driesch's later studies on the 

 isolated blastomeres of sea-urchins indicate that here, although 

 a definite polarized segregation of material has taken place at the 

 time of the earlier cleavages (directly proved by Boveri's ('01) 

 observations on Strongylocentrotus, indirectly by Driecsh's ('00) 

 comparison of the development of the upper and lower quartets of 

 the 8-cell stage) this segregation Is not only symmetrical with 

 respect to the axis but is also less definite or less complete than 

 In the molluscan egg, — again a difference which finds its nat- 

 ural explanation In the theory of precocious segregation (or dif- 

 ferentiation). I should therefore interpret the differences be- 

 tween the isolated blastomeres of the mollusk and those of the 

 sea-urchin or nemertlne as due to a difference, on the one hand. 

 In the pattern, on the other hand in the degree, of segregation. 



It is hardly necessary to point out that the foregoing conclusions 

 will in large measure reconcile the apparent conflict between the 

 fact of cytoplasmic prelocallzation and the continually Increasing 



