210 



PRINCIPLES OF ANIMAL BIOLOGY 



Spermatogenesis. — This name is given to the maturation of the male 

 germ cells. As soon as the spermatogonia reach the end of their mvilti- 

 plication period, that is, as soon as they have divided by ordinary mitosis 

 for the last time, and are ready to initiate the changes included in matura- 

 tion, the cells are known as primary spermatocytes. The history of these 

 cells in their further development is illustrated in Fig. 164, to which con- 

 stant reference should be made throughout the following account. 



During all of their history up to this time, the germ cells contain the 

 same number of chromosomes as any other cells of the body. That num- 

 ber, barring differences in the sexes, is constant for the species. Now, 



■ms 



Fig. 163. — Origin of the germ cells of a vertebrate animal. Diagram of a cross section 

 of the body of the embryo, showing the germ cells originating in the endoderm of the intes- 

 tine and migrating as shown Ijy the arrows to the later position of the reproductive bodies, 

 c, ccelom; en, endoderm of intestine; gc, germ cells; ge, germinal epithelium which later 

 covers the gonads and from whicn the germ cells issue; i, intestine; m, myotome, or muscle 

 segment; 7ns, mesentery; nc, neuial crest, from which nerves and ganglia develop; nd, 

 notochord, forerunner of the backbone; s, spinal cord. 



in an animal descended from two parents, these chromosomes, with cer- 

 tain exceptions that may for the present be ignored, come in equal num- 

 bers from the father and the mother. Half of the chromosomes in any cell, 

 whether somatic or immature germ cell, may therefore be designated 

 paternal, the other half maternal. These chromosomes may look pre- 

 cisely alike, and may in fact be exactlj^ alike; the terms paternal and 

 maternal refer only to their source, not to their nature. 



In the early stages of maturation, the spermatocytes grow considerably 

 in volume. In these stages also the chromosomes come together in pairs. 

 Each pair is composed of one paternal and one maternal chromosome. 



