314 ELEMENTS OF BIOLOGY 



of great importance in determining the nature of the inheritable 

 characters to be conveyed by the mature sperms. Cells undergoing 

 these changes are known as primary spermatocytes; the stage is 

 marked by an increase in nuclear size and an absence of cell 

 division. 



The nuclear material at the close of the telophase of the last 

 spermatogonia! division is similar in arrangement to that in any 

 nucleus that has undergone a recent mitosis. At the beginning of 

 the primary spermatocyte stage the chromatin becomes organized 

 into a long thread, the spireme. Presently this spireme appears to 

 be a double thread an.d then severs into a number of double 



S 

 chromosomes, the number being one-half the somatic number, or — • 



2. 

 At this time these chromosomal bodies are in reality paired chromo- 

 somes, p and m being now united for the first and only time during 

 the life history of the animal. The paired chromosomes are during 

 this period either fused side by side or entwined together, and as 

 they separate during the later stages of the primary spermatocyte, p 

 contains a part of m and w contains a part of p. In other words, 

 substances have crossed over and been exchanged between the 

 two members of a pair. This contact period is known as synapsis. 

 When the chromosomes separate, p is no longer the original p 

 but a new mixture of chromosomal materials which may be desig- 

 nated as P; m is no longer w, but another mixture, not identical 

 with P, which we may designate as P\ 



The primary spermatocyte now passes through a modified meta- 

 phase in which all P chromosomes are on one side the equatorial 

 olate and all P' chromosomes on the other side. On one side is the 

 mX which we may now call PX and on the other side is the PY. 

 In the succeeding anaphase all P chromosomes and PX go to one, 

 and all P' chromosomes and PY go to the other centrosome. The 

 division of the cell, which now follows, thus divides the chromo- 

 somes so that one of the daughter cells contains one set of the 



