34 THE STRUCTURE OF THE FERTILISED EGG 



chains. The polypeptide chains are probably folded in the region 

 of the chromidia, but stretched in the intervening parts. 



Several investigations, among which those of J. Brachet are 

 prominent, have demonstrated that microsomes can be isolated 

 from damaged cells by ultracentrifugation. These microsomes 

 may be identical with Monne's chromidia. Apart from ribo- 

 nucleic acid, they contain sulfhydryl compounds, calcium, 

 magnesium, phosphatids, and numerous enzymes. Probably 

 these microsomes are important centres of metabolism and 

 protein synthesis. It seems likely that they are able to multiply 

 by division. 



At the egg surface, we find a layer of a more solid consist- 

 ency, the egg cortex. In the majority of cases, this is already 

 formed by a peripheral condensation process before fertilisation 

 takes place. It is a few /x thick, at most. Investigations by 

 Monroy (1947) and Monne have shown that the cortex of sea 

 urchin eggs probably consists of alternating protein layers and 

 lipid lamellae. The filamentous polypeptide chains are parallel 

 to the surface of the egg, whereas the rod-shaped lipid molecules 

 are at right angles to it. During fertilisation and the first 

 cleavages of the egg, rhythmical changes take place in the 

 structure of the cortex. 



All this tends to show that the composition of the egg cyto- 

 plasm is very complicated indeed, but that its structures do not 

 adumbrate the structure of the future embryo. They are entirely 

 in the nature of an intensive multiplicity, as defined above 

 (p. 3). 



Apart from the egg cytoplasm, however, the fertilised egg 

 also contains a nucleus, originating from the fusion of the 

 nuclei of egg and sperm. During cleavage this nucleus also 

 divides into a number of cleavage nuclei which find their way 

 into the various blastomeres. Could it not be that the egg has 

 a complicated spatial structure, localised, however, not in its 

 cytoplasm but in the nucleus, in which, after all, the spatial 

 multiplicity of the future embryo might be preformed in some 

 way or another? 



Weismann advocated this hypothesis in 1892. In his opinion, 

 the nucleus of the fertilised egg contained the id, a three- 



