ORGANO-GENESIS 385 



surrounding medium and in the cytoplasm as well. (It has been 

 proved experimentally that hanging drops coalesce if their surface 

 films are soluble in the drops themselves, while they separate if 

 the film is soluble in the surrounding medium.) 



After the second segmentation we have four cells each a 

 diminutive whole egg of identical composition and each capable 

 of developing into an embryo. 



The third division brings about an unequal division of the 

 cell material. Three zones can be recognised in all the cells up 

 to this stage. These three zones, viz. (A) a clear cap at one 

 pole ; (B) a zone with a pigmented surface ; and (C) a large 

 unpigmented zone, each give rise to a definite part of the deve- 

 loped egg. Thus up to the third division every constituent of 

 the original egg is present in the segments in the original pro- 

 portions. The third division is equatorial and cleaves the cell- 

 mass unequally. Four cells are formed containing little or no A, 

 and the other four cells contain only a trace of C. These latter 

 form at the next division four very small cells called micromeres 

 mostly of A, and four larger pigmented cells (intestinal cells). 

 Eight large cells (ectodermal cells) are formed from material which 

 is mostly C, but contains some B. 



The cell division proceeds and the tiny cells all gather at the 

 surface of the egg surface adsorption. Soon after the tenth 

 division, when the number of cells is theoretically 1024, the 

 processes of invagination and differentiation begin. 



Organo-genesis. Various parts of the egg give rise to various 

 organs. Always the same organ is formed from the same part. 

 This means that the apparently homogeneous protoplasm is 

 heterogeneous, i.e. contains colloidal matter in different parts of 

 maybe a specific chemical nature certainly in a specific physical 

 state. One cannot as yet say why certain cells grow in certain 

 directions or why certain organs should be evolved from certain 

 cells and only from these cells, but certain mathematical and 

 physical phenomena have been observed in this connection. 



If one postulates, in the first instance, the presence in the egg 

 of regions denser than others, for example, one can imagine as 

 a result unequal growth in various parts. Unequal growth sets 

 up strain ; and strain, as we saw in Chap. XVI., influences the 

 external form and internal structure of organs. This can be 

 demonstrated experimentally by building an artificial blastula of 

 little pellets of dough containing different quantities of yeast. 

 The unequal growth of the various pellets will set up mutual 



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