Chemistry of Living Matter and Cell Division 



103 



separates into its two component parts, one going to each daughter 

 nucleus. Thus the second polar body is formed which is also pinched 

 off from the egg cell proper. Often the first polar body again divides to 

 form two tiny cells, but none of the polar bodies perform any actual 

 known function for the organistn. From this account we note that 

 four cells have formed from the primordial egg cell — the egg propel 

 and three polar bodies. Two of the polar bodies are the result of me 

 first polar body dividing in turn. / 



It is of great importance to note that the order of development may 

 change in different species. For example, some polar bodies never 

 divide, while in some species maturation takes place before, and/m others 



after fertilization. / 



We shall see in oar study of 

 plants that this reduction-division is 

 not confined to the animal world. 



The male ceJf — the sperm — 

 passes through^ similar changes to 

 that described for the egg cell, ex- 

 cept that there are no polar bodies 

 formed. 



In Biology we always think of 

 the reproductive cells as the germ 

 plasm which alone carries on from 

 parent to offspring all things that 

 can be inherited. It must, therefore, 

 follow that there is something in the 

 germ plasm which determines what 

 the offspring is to be. These de- 

 termining factors must be in the 

 chromosomes because it is only the 

 chromosomes which pass from par- 

 ent to child. But there can be a con- 

 siderable "change-about" of the 

 chromosomes. For example, if we 

 have four chromosomes numbered 

 1, 2, 3, 4, either 1 and 2 may be 

 thrown out in the reduction division, 

 thus leaving 3 and 4; or 1 and 3 may be thrown out, leaving 2 and 4; 

 or 2 and 3 may be thrown out, leaving 1 and 4; and so on. 



If it be remembered that quite a number of combinations can be 

 made in this way in both the egg and the sperm, it is readily understood 

 that several times this number of combinations can be brought about 

 by a mingling of sperm and egg after fertilization, when the reduced 

 sperm cell unites with the reduced egg cell. When we come to the study 

 of Genetics, we shall enter into this phase more thoroughly. 



Fig. 33. Maturation of the Egg of Cyclops 

 (the full number of chromosomes is 

 not shown). 

 A, chromosomes already split longitudinally; 

 B, chromatin masses with indication of trans- 

 verse fission to form the tetrads; C, the young 

 tetrads arranging themselves- on the first polar 

 body spindle; D, tetrads in first body spindle; 

 E, separation of the dyads in the same; F, 

 position of the dyads in the second polar body 

 spindle, the first polar body being really above 

 the margin of the egg. (After Ruckert.) 



