LIMITS OF DIVISIBILITY OF LIVING MATTER. 57 



transform the material of the different cleavage cells unequally 

 during segmentation. The consequence might be that an iso- 

 lated cell of the eight-cell stage could no longer develop into 

 a perfect embryo, while the eighth part of the same ovum before 

 segmentation was potentially able to produce a whole embryo. 

 Therefore the method of isolating a cell of the segmented 

 egg could not be relied upon for our purpose. The second 

 method is one that I described in the Biological Lectures of 

 last year, and is as follows : The eggs of sea-urchins were 

 brought into sea-water that had been diluted by the addition 

 of about one hundred per cent of distilled water. The contents 

 of the egg took up water very rapidly, and the thin membrane 

 of the ovum burst in one or more places. The protoplasm 

 which escaped from the opening thus made assumed the shape 

 of a sphere, and at first remained connected with the protoplasm 

 within the membrane. As soon as the eggs were brought back 

 into normal sea-water they began to segment, segmentation 

 taking place in the extra-ovate as well as in that part of the 

 protoplasm that remained within the membrane. Later on 

 either the extra-ovate and the ovum formed a single blastula, 

 or the extra-ovate and the protoplasm that had remained in 

 the ovum formed two separate segmentation cavities and the 

 egg gave rise to twins. In some cases the twins remained 

 grown together, but more often they became separated. When 

 more than one extra-ovate was formed, three and more embryos 

 would be obtained. It sometimes happened that even when 

 there was only one extra-ovate, crevices would be formed in 

 the substance during segmentation in such a way as to produce 

 more than two embryos. — When the eggs were made to burst 

 before segmentation had taken place, only one nucleus was 

 present, and this was located sometimes inside the ovum, some- 

 times in the extra-ovate. I showed last year how the nuclear 

 material becomes distributed throughout the whole protoplasm 

 of an ovum which has been made to burst. 



3. In these experiments the size of the extra-ovate naturally 

 varies. This being the case, it is evident that these extra- 

 ovates may give the answer to our question as to what is the 

 order of magnitude of the smallest quantity of egg substance 



