LIMITS OF DIVISIBILITY OF LIVING MATTER 325 



mo iv than two embryos from an cirif. It' the e<_f-s HIV made 



/ oo 



to burst before segmentation begins, only one nucleus is 

 present, and in eonse<|uenee either the contents of the e^- 

 or the extraoyate must be without a nucleus. I have already 

 mentioned in my earlier papers that in the course of the 

 segmentation nuclear material gets into that portion of the 

 protoplasm which was originally free from it. Occasionally 

 the extraoyate is cut off before nuclear division occurs. 

 Nevertheless, cleavage occurs. From the observations of O. 

 and R. Hertwig and of Boveri we may assume that in these 

 cases a spermato/oon has entered the protoplasm. The 

 nuclear material which is introduced in this way suffices to 

 inaugurate the process of cleavage. 



3. In these experiments it is natural, of course, that the 

 extraovate, as a rule, does not contain exactly one-half the 

 mass of the egg. Those cases in which the extraovate and 

 the contents of the egg differ greatly in size are well adapted 

 to decide how large an amount of the egg-substance is just 

 sufficient to give rise to a normal pluteus. I followed the 

 development of selected individual eggs with their extra- 

 ovates in a drop of water contained in a moist chamber. I 

 also examined very carefully from day to day cultures of 

 such eggs kept in large vessels, and determined the size of 

 the smallest plutei. Finally, I studied also from day to day 

 the fate of these small fragments. The results of these ob- 

 servations, which I pursued uninterruptedly for two months 

 last year and again for two months this year, were very defi- 

 nite, and may be expressed as follows: 



it) The smallest normal plutei which arose from fragments 

 of an egg were linearly about half the size of the plutei aris- 

 ing from a whole egg of the same culture. Their volume- 

 their density being considered the same was therefore about 

 one-eighth of that of a normal pluteus. 



l>) Smaller fragments of an egg than these developed into 



