294 WATASE. [Vol. IV. 



with this particular case. My reasons are as follows : I have 

 shown that the area which is the seat of a particular variation 

 either in the behavior of the nucleus or of the cytoplasm, such 

 as the peculiar fusion or the incomplete division of the cyto- 

 plasm, or in the velocity of the caryokinetic division, can always 

 be traced to one single segment in a still earlier stage. In other 

 words, when an area consisting of a small number of cells shows 

 a peculiar modification, that area can always be traced to a single 

 cell in a still earlier stage, from which it has descended. When 

 analogous phenomena of variation occur on both sides of the 

 bilateral axis of the blastoderm, the area where such phenomena 

 occur can be traced to one corresponding cell on each side of 

 the axis, which came into existence at the same time on both 

 sides. Or when the variation occurs only on one side, or at one 

 spot, this area of variability is made up of the descendants of a 

 single cell. 



It is difficult to suppose that the environment affects one 

 quadrant of the blastoderm at the four cell stage different from 

 another ; or how the external conditions can affect the corre- 

 sponding segments on each side of the blastoderm and divide 

 their nuclei with three asters, when all the rest of the nuclei 

 are divided with two, it is difficult to imagine. 



The cause of unequal cleavage in the various cases we have 

 examined appears to me to be an internal one, due to the pecu- 

 liarities of the particular protoplasmic structure which composes 

 the segment or segments. When, therefore, the right and left 

 halves of the bilateral blastoderms show a difference of velocity 

 in their cleavage, I believe it is due to the slight qualitative 

 inequalities induced by the first division, — inequalities which 

 appear more and more exaggerated as the cleavage process 

 advances. 



These facts seem to point to two conclusions : — 



i) That the earlier cleavage processes are more fundamental, 

 and, from the morphological standpoint, more significant than 

 the later ones. 



2) That, as I have already mentioned, since the eggs from 

 the same animal show similar variations in cleavage, such a 

 tendency to vary may become hereditary. This conclusion is, 

 however, a provisional one. 



Clark University, Worcester, Mass., 

 November 19, 1890. 



