<j2 THE GENERATIVE ORGANS. 



cord is seen as a prolongation of the narrow end ; Brandt 

 observed a similar change of form in the testis of the larva 

 of Pieris Brassicae. The principal changes it now undergoes 

 are, the whole organ grows rapidly in size, and the genital cord 

 becomes hollow and forms the vas efferens ; which is seen at a 

 very early period to consist of an external coat, similar to and 

 continuous with the mesoblastic capsule of the testis ; this 

 becomes the peritoneal coat, and exhibits numerous closely 

 adherent multi-nucleated cells young fat-cells. 



The capsule of the testis extends inwards in the form of fine 

 septa of adenoid tissue, and divides the testis into numerous 

 loculi. These septa in the Blow-fly are exceedingly thin and 

 delicate ; in Pieris, according to Brandt, the testis divides 

 into three or four lobules by the ingrowth of the external 

 capsule. The septa in Musca are the only indications of lobu- 

 lation which occur in this insect. These loculi are all filled 

 with small round cells which are at first similar to each other 

 throughout the whole testis, but these do not long remain so ; 

 those nearest to the duct are rapidly converted into young 

 spermatozoa, whilst those at the fundus remain unchanged, 

 even after the insect escapes from the pupa. 



The paragonia are apparently developed as diverticula from 

 the point where the vasa efferentia unite. In the earliest stage 

 of development there is apparently no trace of these organs, 

 and they only subsequently make their appearance as short 

 csecal diverticula of the vas deferens. In the pupa of the 

 eighth day the paragonia and vasa efferentia are each nearly 

 i mm. in length, and the vas deferens is only slightly longer. 



d. Spermatogenesis. 



The formation of the spermatozoa in the testis takes place 

 in the pupa, and the male insect is sexually adult a few hours 

 after leaving the pupa case. Owing to the fact that the cells in 

 the testis do not undergo simultaneous changes all the stages 

 of spermatogenesis can usually be observed in male insects 

 a few hours after their escape from the pupa, and the process 



