1896.] Embryology. 245 



formation of the blastoderm. 



" not in accordance with any previous location of a nucleus, but inde- 

 pendently." Dr. McMurrich thinks that " this phenomenon seems to 

 demonstrate that cytoplasmic differentiation may occur independently 

 of definite nuclear influence." He immediately adds, however, that 

 " he, of course does not mean to assert that the nuclei may not possess 

 a coordinating or i w n * •,■■< ■"•fion upon the cytoplasm, but that they 

 are directly responsible for the segregation or concentration seems to him 

 an unwarranted assumption." 



It is difficult to understand just what is meant by these statements. 

 The remarkable concentration of the peripheral protoplasm of the ovum 

 of Porcellio toward the definitive ventral side, independently of any 

 previous location of nuclei, is notewothy. Does it, however, "demon- 

 strate cytoplasmic differentiation independent of definite nulcear influ- 

 ence?" Can this movement of protoplasm, even toward a definite 

 point, be correctly spoken of as differentiation and compared with the 

 specialization in the cytoplasm of certain protozoa? Having in mind 

 the condition of the ovum when this phenomenon takes place, is it not 

 possible that the movement may be the result of nuclear influences from 

 the center, acting through the central network on the periphal proto- 



Again, if the phenomenon demonstrates cytoplasmic differentiation 

 independent of definite nuclear influence, why does the author add that, 

 '• he does not mean to assert that the nuclei may not possess a coordi- 

 nating action upon the cytoplasm ? " There seems to be a contradiction 

 in these two statements, which may destroy the force of the argument. 

 It should also be remembered that in Jsera, and in the other Isopods 

 studied, there is a contraction of the blastoderm cells toward the ventral 

 surface. Here the nuclei, as well as the cytoplasm, of the blastoderm 

 are evidently directly involved. Perhaps the precocious segregation of 

 cytoplasm ventrally in Porcellis is but an early appearance of this pro- 

 cess. If it be admitted that the nuclei possess coordinating influences 

 on the cytoplasm, how can it be claimed that in the case of the highly 

 differentiated protozoa such influences were not active during the 

 differentiation ? 



Another point discussed is the extent of external influences and their 

 action, in holoblastic and in centrolecithal ova like those of Jiera. The 

 conclusion reached from a review of the facts of segmentation is that " the 

 cleavage form of Jsera is determined entirely by intrinsic conditions." 

 The phenomena of segmentation " leave us no choice but to refer the 

 vis essentialis which determines the direction of the Karyokinetic 



