ORGANIC CHEMISTRY AND CELL DIVISION 



103: 



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

 the 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 di- 

 vide, while in some species maturation takes place before, and in others 

 after fertilization. 



We shall see in our study of 

 plants that this reduction-division is 

 not confined to the animal world. 



The male cell 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 \vhich 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, as it is only the chro- 

 mosomes which pass from parent to 

 child. But there can be a consid- 

 erable "change-about" of the chro- 

 mosomes. For example, if we have 

 four chromosomes numbered like 

 this: 1, 2, 3, 4, either 1 and 2 may 

 be thrown out in the reduction divi- 

 sion, 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 there can be several times this number of combinations brought 

 about by a mingling of sperm and egg after fertilization, when the re- 

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

 the study of Genetics we shall enter into this phase more thoroughly^ 



It will, of course, depend upon what characters are thus carried by 

 the two mating chromosomes as to what characters the new organism 

 will possess. It is this assorting and rearrangement of chromosomes 

 which is in all probability the cause of variations within a given species. 

 This is.by.no means the same as saying that it is the cause of new 

 species. This distinction must be kept clear. 



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 Riickert.) 



