304 THE DEVELOPMENT OE THE NYMPH. 



that section of my work I was unacquainted with the masterly 

 memoir of \'an Rees, and I would here remark that my con- 

 clusions agree so closely with his that, without this disclaimer, 

 it might be supposed I had derived them from him without 

 acknowledgment. 



I shall perhaps do best to give a translation of the thesis 

 which ^'an Rees supports in his memoir. After giving the 

 history of the views of his predecessors, he says [147, p. 22] : 

 ' It appeared to me that there is only one possibility which 

 leads to a complete solution of the problem before us, that 

 the imaginal discs are not only ectoderm, but are invaginations 

 of the ectoderm itself. I can onl\- understand the manner 

 in which the Muscidas are developed by supposing that the 

 ancient progenitors of the Flies had imaginal discs, which, 

 like those of the Tipulidse, lay in immediate relation with 

 the larval hypoderm : and that in later generations these 

 were continually drawn more and more deeply into the maggot 

 until they assumed their present positions, 



' The fact that each imaginal disc in Corethra is supplied by 

 a nerve and a tracheal vessel affords us a clue to the manner 

 in which the relation of the discs to the hypoderm has been 

 maintained. Although at length the imaginal rudiment may 

 appear as a mere appendage of the nerve or tracheal vessel, it 

 has nevertheless a neck or hollow pedicle closely applied to the 

 nerve or trachea. This is readily seen when the neck is short, 

 but in the extreme case we have probably to deal only with a 

 difference of degree, and not of kind, so that we may conclude 

 that the direct relation between the disc and the hypoderm is 

 always maintained, and that the insertion of the pedicle into 

 the hypoderm indicates the point where the disc must have 

 lain in the ancestral form. 'The object of my researches has 

 been to demonstrate this postulated connection between the 

 disc and the hypoderm.' 



Van Rees brings no embryological evidence to support his 

 thesis, and merely remarks that he has traced the pedicles 

 of the wing-discs into the hypoderm in the half-grown larva, in 



