82 SEX AND HEREDITY 



This question can best be answered by considering a 

 concrete case of syngamy and early development of 

 an animal, and we will choose as our example the 

 little, almost microscopic, shrimp-like creature Cyclops. 



Fig. 46, A, shows the egg or macrogamete of Cyclops 

 immediately after the entry of the spermatozoon (m), or 

 microgamete. The great bulk of the egg consists of 

 protoplasm densely packed with yolk. Like all cells, 

 the egg contains a nucleus (/), which is seen close under 

 the egg shell. It has travelled to this position prior to 

 dividing into two and getting rid of one of the halves. 

 This at once introduces us to one of the most important 

 features of the hereditary substance. If, as we are 

 bound to assume, this substance has a definite structure, 

 it is obvious that there must be some method of reducing 

 it by half once in each generation, since otherwise it would 

 be doubled by the fusion of gametes in each act of syngamy. 

 Now we find that a halving of the nuclear material of 

 the gametes takes place at or before syngamy (in the case 

 of the spermatozoon this halving has already taken place 

 before it enters the egg). This fact in itself points there- 

 fore to a probability of the nuclear substance being the 

 material substratum of heredity. 



In Fig. 46, B, we see both the male and female nuclei 

 travelling in towards the centre of the egg, the nuclear 

 material (p) discarded from the female nucleus being 

 conspicuous just under the egg shell. It can be seen in 

 this position for a long time in development, but itself 

 plays no part in this process, being indeed dead and 

 rejected. 



Fig. 46, C, shows the two nuclei (m and/) closely applied 

 to one another in the centre of the egg, and we notice the 

 important fact that they are now equal in size. Fig. 



