48 



INVERTEBRATE MORPHOLOGY. 



The transformation of the germ-cells present in an era 

 bryo into spermatozoa is usually a somewhat complicated 

 process. In the Round-worm Ascaris, in which it retains 

 somewhat primitive characters, the process closely resem- 

 bles what takes place during the maturation of the ovum. 



PlG. 22. DlAGKAMS TO ILLUSTRATE THE MATURATION OF THE SPEKM-CELL- 



A = division of the spermogone. 

 B division of the two spermocytes. 

 C = the four spermatids. 



D, E conversion of a sperinatid into a spermatozoon. 

 F fully developed spermatozoon. 



The embryonic germ-cells (spermatogones, Fig. 22, A) undergo 

 karyokinetic division, the number of chromosomes being, as 

 in the ovum in the division which results in the formation of 

 the first polar globule, twice that which is characteristic for 

 the species. They do not undergo longitudinal division, and 

 one half of them passes into one of the daughter cells (sper- 

 matocytes) and the other half into the other, so that these two 

 cells possess the number of chromosomes characteristic for 

 the species. A division of these daughter cells (Fig. 22, B) 

 immediately takes place without a return to the resting stage, 

 and unaccompanied by a longitudinal division of the chromo- 

 somes, so that four cells (spermatids, Fig. 22, 6') are formed, 

 each of which contains only half the typical number of chro- 

 mosomes, and each one of these cells becomes a spermato- 

 zoon. This process is comparable step by step with the 



