347 



of the sternal region of the ninth imaginal disc continue to 

 proliferate rapidly, and grow inward, into the eighth seg- 

 ment; the extension is very rapid, so rapid, indeed, that after 

 about six hours the invagination has extended forwards as 

 far as the posterior border of the sixth segment; the process 

 continues, and does not cease till the invagination has e^x- 

 t ended, along the mid-ventral region, well into the fifth 

 abdominal segment. 



Early in the process of invagination a cavity is developed 

 in the mass of cells; and at the terminal (anterior) end this 

 cavity dilates, to form a small sac, the vesicula seminalis 

 (fig. 27), which is, therefore, formed from deeply invaginated 

 epidermal cells, and is in no way to be regarded as a meso- 

 dermal structure. In the six-hour pupa, though already 

 clearly defined, it has, nevertheless, not attained a very 

 pronounced size; but some twelve hours later, it is quite a 

 prominent bulbous dilatation at the anterior end of the penis. 



The penis is thus a structure composed of the sternal 

 portion of the ninth segment, and the appendages of the 

 tenth, and the whole organ is produced simply by a massive 

 ingrowth of cells of the ninth segment, forwards, along the 

 ventral body wall. 



Already in the earliest pupae the transference of the 

 appendages of the tenth segment to the sternum of the ninth 

 is clearly visible, but it is not till well within the second day 

 that the distinct development of a joint separating the two 

 is evident. Early in the pupa the two appendages unite to 

 form a simple tube, but exactly how this takes place I have not 

 been able to observe. 



The penis is, then, a simple tube, consisting of two por- 

 tions, a proximal, representing the sternum of the ninth 

 segment, and a distal shorter portion, developed from the 

 appendage of the tenth segment. The distal segment is seen, 

 in the pupa, to be invaginated into the ninth; and both the 

 segments are provided with a pair of long tendons, which serve 

 to withdraw the distal joint into the one preceding it, and, 

 finally, the whole structure into the abdomen. In this con- 

 dition the organ is seen during later pupal life, and the ventral 

 termination of the abdomen of the male, though really so 

 totally different from that of the female, has, nevertheless, a 

 curious resemblance to it. This is due to the fact that develop- 

 ment of the male copulatory organ is mainly a process taking 

 place within the abdomen, after the pupa has been formed. 



This internal development of the tenth and ninth seg- 

 ments is accompanied by a number of changes in other 

 abdominal segments, which result, in part, in the formation 

 k2 



