CHAPTER VII 



The topogenesis of the embryo 



In the foregoing chapters we have seen how a series of processes 

 transforms the initially fairly homogeneous structure of the 

 egg into a system with a considerable degree of spatial multi- 

 plicity. First, the primary co-ordinate system of the egg, 

 expressed in its polarity and symmetry, gave rise to the 

 localisation of determining substances. This was followed by 

 the production and local accumulation of new components of 

 the cytoplasm, resulting from the chemical and physical pro- 

 cesses now started in the egg. Differences began to occur, both 

 chemically and physically, among the various parts of the egg. 

 Finally, reactions between nuclei and cytoplasm took place as 

 a consequence of the activation of the genes under the influence 

 of the local cytoplasm. Taken together, all these processes 

 break up the originally fairly homogeneous system into a mosaic 

 of physically and chemically different areas. We have called 

 this the chemodifferentiation of the egg. 



As a rule, chemodifferentiation does not manifest itself very 

 clearly in the egg's external appearance. In exceptional cases, 

 some of the determining substances may be visible externally 

 because they have pigments of different colours; if so, their 

 movements can be followed easily. In general, however, only 

 experiments can inform us of the changes that have taken place 

 in the egg. 



Another process, however, also subsequent to fertilisation, 

 and usually more or less simultaneous with the primary chemo- 

 differentiation, can be easily observed. This is the cleavage 

 of the egg, whereby the originally continuous mass of egg 

 cytoplasm is divided into a number of cells. The zygote nucleus 

 also divides a great number of times so that, in the end, the 



