692 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 Brassice. ‘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 
cecal diverticula of the vas deferens. In the pupa of the 
eighth day the paragonia and vasa efferentia are each nearly 
1 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 
