METAMORPHOSIS 



167 



Imaginal Discs. The imaginal discs are portions of the 

 larval hypoderm, detached from continuity with the main body 

 of the integmnent, but connected therewith by strings or pedicels 

 which may be looked on as portions of the basement membrane. 

 Whether these discs, or histoblasts as they are called by Kiinckel 

 d'Herculais/ are distinguished by any important character from 

 other buds or portions of regenerative tissue that, according to 

 Kowalevsky,^ Korschelt and Heider,^ and others, exist in other 

 parts of the body, does not appear to be at present ascertained. 



We give some figures, taken from Weismann and Graber, of 

 the imaginal rudiments existing in the 

 larvae of Muscidae. Although by no means 

 good, they are the best for our purpose 

 we can offer to the reader. Other figures 

 will be found in Lowne's work on the 

 blowfly now in course of publication. 

 Weismann's paper * is now thirty years old, 

 and, when it was written, he was not aware 

 of the intimate connexion the rudiments 

 have with the integument ; this has, how- 

 ever, now been demonstrated by several 

 observers. Pratt states ^ that the formation 

 of the imaginal discs in Melophagus ovinus 

 takes place in the later stages of the em- 

 bryonic development, and after the manner 

 formerly suggested by Balfour, viz. invagin- 

 ation of the ectoderm. 



Both the regenerative buds and the 

 rudimentary sexual glands are known to be 

 derived directly from the embryo ; neither 

 of them undergoes any histolysis, so that Fig. 

 we have in them embryonic structures 

 which exist in a quiescent condition during 

 the period in which the larva is growing 

 with great rapidity, and which when the 

 larva has attained its full growth and is 



Median longi- 

 tudinal section through 

 larva of blowfly during 

 the process of histo- 

 lysis. (After Graber.) 

 Explanation in text. 



disintegrating. 



then 



^ Eeclierches Org. des Vohicelles, 1875, p. 143. 

 2 Zeitschr. wiss. Zool. xlv. 1887, p. 587. 

 ^ Lehrhuch Entivicklungsgeschichte, Spec. Theil. 1890, p. 875. 

 * Zeitschi\ wiss. Zool. xiv. 1864, p. 187. ^ Arch. f. Naturgcs. lix. 1893, 1, p. 



168. 



