724 LIFE : OUTLINES OF GENERAL BIOLOGY 



testes are so like one another that it is difficult for a time to dis- 

 tinguish the young male from the young female. 



Ova and spermatozoa are true cells, complementary to one 

 another in fertilisation, but the spermatozoon has a longer cellular 

 lineage behind it, and the real homologue of the ovum is the sperma- 

 togonium or sperm-mother-cell in the testis. These spermatogonia, 

 sometimes not unlike very young ova, divide and redivide into 

 smaller spermatocytes. There is often an interesting resemblance 

 between the ways in which spermatogonia divide and the modes of 

 segmentation in ova; thus a sperm- morula is not unlike an ovum 

 with partial peripheral segmentation. But it is unlikely that this 

 resemblance means more than that the ways in which a cell can 

 divide are few. There may be several generations of spermatocytes, 

 and eventually spermatids are formed, which then turn into special- 



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Fig. 112. 



A, IMmary Germ-Cell, amoeboid; B', a spermatogonium, which divides into 

 a sperm-morula (C and D'), giving rise to separate spermatozoa (s/>); 

 B, an unripe egg-cell or oocyte; C, the egg-cell giving off the first polar 

 body (ipb); D, the formation of the second polar body {2pb); E, the 

 egg-cell with reduced female pronucleus (RFN); F, fertilisation, the 

 intimate orderly union of the two reduced sex-nuclei, namely the sperm 

 nucleus (SPN) and the reduced female pronucleus (RFN). 



ised spermatozoa. But at some stage in the spermatogenesis, there 

 is a reduction or meiotic division, and this often occurs at the 

 formation of the second last crop of spermatocytes. We shall return 

 to this meiosis in connection with the ovum, in whose maturation a 

 similar process occurs, but the large fact is that the normal number 

 (n) of chromosomes is halved, so that in the last crop of spermato- 

 cytes there is in each cell half that number ( - ). If the spermatocytes 



with the reduced or haploid number of chromosomes should divide 

 again, they divide in the ordinary equational or mitotic fashion, so 



that the spermatozoa have still the - number. In the many diverse 



types there are differences in the details of spermatogenetic reduc- 

 tion, and as regards the stage at which it occurs; but in most cases 



