52 CLEAVAGE II 



a part in the ordering of the pattern appear to be, sometimes, 

 the physical properties which they possess as liquids ; for in 

 spirally segmenting eggs, where never more than three surfaces 

 of contact between adjacent blastomeres intersect in one line, 

 making angles of 120 with one another, the cells are merely 

 obeying the laws of surface-tension as enunciated by Plateau 

 for systems of drops, for example soap-bubbles. In the other 

 types of cleavage, it is true, these capillary laws are not obeyed, 

 but even here as Roux has shown it is possible to cause drops 

 of oil to imitate the arrangement of a radial system by enclos- 

 ing them in a boundary ; in the radial eggs the membrane 

 may represent such a boundary. 



More than this need not be said at the moment since these 

 matters may be found discussed in the text-books. 



3. Nuclear and cell- division continue of course throughout 

 the life of the organism ; and during all but the earliest stages 

 of development they are accompanied by the processes of 

 growth and differentiation. There is, however, an earliest 

 stage of all in which the material of the ovum is simply cut 

 up into small pieces, the cells, without the concurrence of any 

 growth or of any differentiation other than the formation of 

 the segmentation cavity, and such physical and chemical altera- 

 tions as may be taking place in the blastomeres under perhaps 

 the influence of their nuclei. This early stage is the stage of 

 segmentation, and we have now to discuss the nature of those 

 causes which bring segmentation as such to an end and deter- 

 mine the beginnings of differentiation. 



Following out an idea which originated with Richard 

 Hertwig, Boveri has suggested that initially the cytoplasm is 

 too large for the nucleus, but that by the process of nuclear 

 and cell-division the ratio of plasma to nucleus is reduced 

 until it reaches a certain value, the attainment of which 

 marks the end of cleavage, and the commencement of other 

 processes. 



The evidence adduced by Boveri in support of this hypo- 

 thesis consists in the experimental demonstration that at a given 

 stage of development the ratio in question has always a given 



