804 MK. J. a. 11. FUEW ON THli LAUVAL 



, IJelow the anterior region of the pliarynx there are two 

 extremely small discs which appear to consist of solid longitudinal 

 rods of embryonic cells lying close together near the median line. 

 They are attnched posteriorly to the hypodermal sheet uniting 

 the maxillary discs below the pharynx : they are the labial discs. 

 Their peduncles are attached anteriorly to the median ventral 

 hypoderm of the head immediately posterior to the mouth- 

 opening and some little distance anterior to the opening of the 

 salivary duct in the pharynx. 



I have found no mandibular imaginal discs. These are also' 

 absent in the Blow-Fly (13) and the House-Fly (4). According 

 to Lowne (p. 83) Kunckel d'Herculais described and figured 

 mandibular discs in the re.sting larva of Voliicella, but I have 

 been unable to iind either the liguro or the statement in Kunckel 

 d'Herculais' work (12). 



Imaginal J)iscs of the External Genitalia. 



Immediately in front of the anus there are two imaginal sacs 

 lying against the ventral hypoderm in the median line, each 

 .sac containing two imaginal discs, one on each side in its anterior 

 region. The postei-ior sac is the larger; it has a very short, 

 transversely broad peduncle, which opens out anteriorly into a 

 small imaginal sac, in whose antero- lateral regions lie the two 

 discs. The anterior imaginal sac is similar to, but lather smaller 

 than, the posterior. Both sacs lie in the naid-ventral region of 

 the eleventh body-segment (8th abdominal). The posterior disc- 

 develops before the anterior one, being well formed when the 

 latter is a mere hypodermal thickening. It is possible that 

 differences exist in the formation of these discs in S and $ larva,. 

 which would enable one to detect the sex of a larva, but such 

 differences have not yet been observed. 



From the examination of two specimens in which the anterior 

 disc was in the form of a hypodermal thickening, I am inclined 

 to believe that the paired discs are from the commencement of 

 their formation enclosed in a single common imaginal sac, and 

 that this single sac with its single peduncle is not formed by the 

 fusion during later stages of larval life of two originally separate 

 discs. The evidence on this point, however, is by no means 

 conclusive. 



Imaginal Discs of the Abdominal Segments. 



In the most mature larva examined the abdominal discs were 

 in the form of two small lenticular thickenings in the dorso- 

 lateral hypodei-mis on each side about the middle of each seg- 

 ment. The cells of these discs originate by the amitotic division 

 of an ordinary hypodei'm cell ; the nuclear membrane dis- 

 apjiears, and the nucleoplasm splits up into numerous little masses 

 each of which becomes the nucleus of an imaginal cell. It is 

 possible that the same process goes on in other regions of the 



