84 SEX AND HEREDITY 



46, A, shows the enormous disparity in size between the 

 microgamete, which consists almost wholly of nucleus, 

 and the macrogamete. In many animals this disparity is 

 very much greater, culminating in the birds with their 

 enormous eggs. In the Ostrich it can be calculated that 

 the macrogamete (yolk of the egg) is many billions of 

 times as bulky as the microgamete. The amount of 

 nuclear material in the two gametes is however equal, 

 though owing to the fact that this is in a very concentrated 

 state in the microgamete it may appear less bulky here. 

 As a comparison between Figs. 46, A, and 46, C, shows 

 however, by the time the nucleus of the microgamete has 

 loosened out to the same texture as the female nucleus, 

 the two nuclei are the same size. 



V 



Now it is a matter of general observation that in- 

 heritance throughout the animal and vegetable kingdoms 

 is on the average as strong from the male as from the 

 female parent, and so it is reasonable to suppose that the 

 physical substratum on which inheritance depends is 

 provided in equal quantities by the two parents. Hence 

 we are again led to look to the nuclear material as probably 

 the substance of which we are in search. 



The development of an animal (or plant) from the 

 single celled zygote consists of two main processes : 



(1) The division of this cell into a great number of 

 cells. 



(2) The differentiation of these cells into the various 

 kinds of cells skin cells, muscle cells, nerve cells, etc.- 

 which compose the body. We will follow the first of 

 these processes for a short distance in the development 

 of Cyclops. 



The division of the zygote into the first two cells of the 

 embryo or young Cyclops is shown in Figs. 46, D, E, F. 



