DEVELOPMENT OF THE GENERATIVE ORGANS. 695 



in diameter. Their nuclei vary in size from i ^ to 2 A 6 in 

 diameter. They are arranged in rows very regularly, and 

 the rows are frequently seen to radiate from the centres of 

 the polyblasts. There is no indication of the division of the 

 cell substance between these nuclei. 



Lee has certainly observed the formation of the polyblasts 

 by the repeated division of the nuclei of the spermatospores 

 in Sagitta, and his figures leave no doubt upon the question in 

 my mind, although he says in his text, ' I have been unable to 

 follow the genesis of these polyblasts.' 



From this stage onwards there is no conformity in my obser- 

 vations and those of Lee ; this is undoubtedly due to the fact 

 that the formation of the spermatozoa takes place somewhat 

 differently in the Chaetognatha and the Muscidse. 



The Spermatozoa are formed in packets, each packet corre- 

 sponding to a polyblast. The polyblasts become pyriform, 

 with the thick end towards the surface of the testicle, and all 

 the nuclei are arranged near this thick convex end. The 

 polyblasts then split up into a number of fusiform segments. 

 These are the young spermatozoa ; each at this stage exhibits 

 a distinct vesicular nucleus. 



All my best preparations are stained with picrocarmine and 

 the nucleoli of the young spermatozoa are most definitely 

 stained, whilst the cell substance is sufficiently coloured to 

 render it very distinct. In such preparations there is no trace 

 of a secondary nucleus (Nebenkern). 



The succeeding stages are very simple : the nuclei cease 

 to exhibit a distinct vesicular character, and their size is 

 less than that of the nuclei in the young spermatozoa. The 

 nuclear vesicle has evidently disappeared, leaving only a small 

 ball of chromatin surrounded by a thin layer of protoplasm, 

 which is continuous with the tail. The tails increase rapidly 

 in length, so that they not only extend across the whole 

 diameter of the testis, but are frequently curved ; at the same 

 time they acquire the property of staining deeply, and exhibit 

 a distinct yellow tinge, owing to the action of the picric acid. 

 The size of the bundles of spermatozoa varies considerably, 



