The following diagrams (Figs. 26-27) are presented with 

 the object of visualizing the distribution of the chromosomes 

 in the formation of the gametes or sex-cells. In Fig. 2(j 

 the somatic and primary sex-cells contain but one pair of 

 chromosomes (one paternal and one maternal). When the 

 reduction process begins (synapsis) in the spermatocyte or 

 oocyte, the chromosomes fuse (Fig. 26, 2), then they di- 

 vide lengthwise and transversely and form a tetrad (Fig. 26, 

 3). By two rapid cell divisions four germ cells are formed, 

 two with a paternal chromosome and two with a materna 1 

 chromosome. 



In Fig. 27 each of the somatic and primary sex-cells 

 contains two pairs of chromosomes (two paternal and two 

 maternal). At synapsis they fuse in pairs (Fig. 27, 2), 

 and later form two tetrads. In the two rapid cell divisions 

 by which the germ-cells are formed, the separation of the 

 chromosomes can take place in two ways, (Fig. 27 A. and 

 B.) and with equal frequency. Consequently the germ 

 cells will be of four kinds according to their chromosome 

 content. 



When three pairs of chromosomes are present in the 

 romatic and primary sex-cells it can be readily shown in the 

 same manner that there will be three tetrads formed after 

 synapsis, and that in the two subsequent divisions the se- 

 paration of the chromosomes can take place in four ways 

 and with equal frequency. Consequently the germ-cells 

 will be of eight kinds according to their chromosome con- 

 stitution. 



Exercise. — Make a diagram to illustrate the distribu- 

 tion of the chromosomes in the formation of the sex-cells 

 when the somatic and primary germ-cells contain three 

 pairs of chromosomes. 



The primary germ-cells which are early set apart from 

 the soma-plasm in the developing zygote rapidly increase 

 in number and size. In the maturation stage two success- 

 ive divisions of the spermatocytes and oocytes occur and 

 four cells result, which become the sex-cells, the sperms or 

 eggs. In the case of the spermatocytes the divisions are 

 equal and four sperm-cells are formed, but in the oocytes 

 the divisions are unequal with regard to cytoplasm and but 

 one perfect egg is formed, the other three being thrown off 

 as polar bodies. (See Figs. 6 and 24). 



Although the nuclear divisions at maturation are in- 

 direct or mitotic, they differ from the usual form of mitosis. 



135 



