THE CELL-DOCTRINE 



129 



acs. 



aes. 



Fig. 14. Spermatozoa. A and B, human spermatozoon, two views showing 

 flattening of head (nucleus); C, stage in development of spermatozoon of 

 guinea-pig, showing what is more obviously a cell; D, diagrammatic 

 figure of fully formed spermatozoon of guinea-pig. Acs, acrosome; a /., 

 axial filament; cy, cytoplasm; ft, flagellum, n, nucleus; m. p., middle piece. 

 (A and B after Retzius; C and D redrawn from Agar after Meves.) 



item in fertilization is the entrance of a sperm-nucleus and 

 its union with the nucleus of the ovum. 



At first glance the nuclei of ovum and spermatozoon ap- 

 pear disproportionate in size. When, however, the stages of 

 the sperm-cells within the testis (Fig. 14 C) are studied, it 

 appears that the chromatin of the spermatozoon is both 

 quantitatively and qualitatively equivalent to the chromatin of 

 the ovum of the same species. 3 When the primitive male 

 cells are made into definitive spermatozoa, their nuclei 

 become more condensed, but their actual chromatin content 

 remains the same (Fig. 14 C and D). This conclusion is 

 further supported by the fact that the nucleus of the sper- 

 matozoon increases in size after entering the egg, so that 

 before union the male and female pronuclei are again seen 



3 The only exceptions to this appear to be the cases where the chromosome 

 formulas of the ovum and spermatozoon differ because of the presence of the 

 sex-determining chromosomes which occur in certain species. See: page 204 

 of the present volume. 



