FERTILISATION 480 



Loeb is inclined to believe thul the spermatozoon removes 

 from the egg a siibstanee or condition which inhibits or prevents 

 the process of development. 



On the otiicr hand, it is conceivable that tiie entry of the 

 spermat()ZO()n increases the free energy of the now fertilised o\um. 

 The potential energy of the system cannot be ntilised withont 

 the employment of a small quantity of free energy. This (piantity 

 of free energy may be extraordinarily small as long as it is sullicicnt 

 to start the series of reactions which, once started, arc niito- 

 catalytic. 



One result of the entrance of an effective spermatozoon into 

 an ovum is an acceleration of the processes of oxidation, i.e. 

 metabolism begins, and the various phases of development can be 

 followed by the same calorimetrie methods (direct and indirect) 

 adopted in the study of the energy exchanges of the mature 

 organism. 



Differentiation. The mammalian ovum is holoblastic, that is, 

 undergoes coiuplete segmentation, and forms a nnilberry-like mass 

 of cells which divides into two sets, viz. a group of large central 

 cells and a layer of small cells surrounding these. That is, there 

 is now 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 gives rise to a 

 definite part of the developed egg. Thus, up to this division 

 every constituent of the original egg is present in the segments 

 in the original proportions. The division now 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 .^, and four larger pigmented cells (intestinal 

 cells). Eight 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 1,024, the 

 processes of invagination and differentiation begin. 



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

 organs. The same organ is formed always from the same ])art. 

 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 



