THE PROBLEMS IN MATING AND IN EGG PRODUCTION 117 



somes composing the chromosomes of the nuclear network 

 that transmit characters good, bad, and indifferent. 



In regard to reproduction it may be said that in mammals, 

 where the young are parasitic upon the mother during the 

 earUer stages of their growth, the egg or germ cells are minute 

 (microscopic in size) and contain only a small amount of yolk, 

 called deutoplasm, that enables them to reach the stage at 

 which they develop the processes for attaching themselves 

 to the wall of the mother's uterus. 



In the bird the contents of the egg form the source of food or 

 nutrients for the developing young and foi more than seventy- 

 two hours after emerging from the shell, and hence are very 

 large in comparison to the size of the animal producing them. 



The sexual cells of the male are very small (microscopic in 

 size) and are produced in the testes or male generative gland. 

 The ova does not possess the power of movement while the 

 male element or spermatozoa are endowed with active move- 

 ment. In the bird they take the form of a long whip, with 

 slender lash or filament. By aid of this flagellum they move 

 about in any liquid they happen to be deposited in in search 

 of the ova. 



In the reproduction of animals or plants there are two sexual 

 cells. The female cell in the animal is called ovum and in plants 

 ovule. The male element is called a spermatozo5n in the ani- 

 mal and pollen (a grain) in the plant. These are known by a 

 general term, gametes or marrying cells. 



The new individual is formed by the union or fusion (yoking 

 together) of a male and female gamete and is called a zygote. 



Therefore the zygote is a double structure, in which com- 

 ponents brought in by each of the gametes remain intimately 

 fused in a form of partnership. 



When later this zygote in turn forms gametes this partner- 

 ship is broken and the process is reversed. In other words, 

 the component parts of the double structure are resolved, with 

 the formation of gametes, into single structures. 



From this the Ufe cycle of a bird may be divided into — First, 

 a period of isolation in the form of a gamete and hving as a 

 single unit awaiting the union with the gamete of the opposite 

 sex to make possible further development; second, the period 



