696 THE GENERATIVE ORGANS. 



and some are seen to be surrounded by a distinct sheath, which 

 occasionally exhibits a large nucleus. These sheaths are pro- 

 bably the remains of the daughter-cells, which have been only 

 partially converted into spermatozoa by endogenous cell for- 

 mation. 



These observations agree with those of Gilson [357], who 

 holds that when the spermatozoa are seen enclosed in a cap- 

 sule, this consists of the remains of a cell of a previous gene- 

 ration ; he further regards each spermatozoon as a distinct 

 differentiated cell, and says that in all other points variations 

 exist in the manner in which spermatogenesis occurs in 

 different animals. I would go even a step further, and think 

 that not only do such variations occur in different animals and 

 groups of animals, but that the process of cell multiplication 

 which precedes spermatogenesis varies under different condi- 

 tions in the same animal. Thus, the size of the polyblasts in 

 the Blow-fly varies within wide limits. In some cases they 

 are comparatively small and contain few nuclei when the for- 

 mation of spermatozoa commences ; whilst others are very 

 large, and contain numerous nuclei. Again, some of the sper- 

 matospores remain enclosed in a distinct nucleated envelope, 

 and others are separated by the exogenous division of the 

 mother-cells. It appears to me that exogenous and endo- 

 genous cell multiplication occur in all stages of the process. 

 Sometimes a spermatospore continues to multiply by exogenous 

 division for a long time ; whilst in a sister-cell endogenous 

 multiplication takes place without previous exogenous division. 

 In the former case small, and in the latter large polyblasts 

 arise ; small bundles of spermatozoa originate in the small 

 polyblasts, whilst large ones are formed in the large poly- 

 Masts. Some of these remain enclosed within the mother- 

 cells, whilst others are apparently free and unenclosed. 



There is no doubt active karyokinetic changes occur 

 throughout the whole process, but I have not attempted the 

 study of the nuclear figures, as I feel that the complexity of 

 the phenomena hitherto described, and the great discrepancies 

 which exist in the statements of various observers, are such 



