334 THE DEVELOPMENT OF THE NYMPH. 



connective tissues or endothelial cells, they arc as a rule 

 temporary structures, and form no part of the adult organism. 

 By mesoderm I mean cellular layers derived directly from 

 the epiblast or hypoblast, or both, the cellular elements of 

 which have never been wander-cells— amoeboid corpuscles— and 

 from which, as a rule, the muscular tissues are developed. 



Whether there are muscle fibres which are developed from 

 wander-cells is, I think, doubtful ; but it is possible that there 

 are, as Hertwig maintains. 



Structure of the Imaginal Discs.— During the evolution of the 

 imaginal discs, the details of their structure become far more 

 apparent than in the earlier stages of their development, and 

 since the issue of the first part of this work in i8go I have seen 

 good reasons for modifying what I then wrote concerning the 

 nature of the mesoblast of the discs (p. 77). What I then 

 regarded as mesoblast I now hold to be parablastic tissue. 



In Plate XVIII., F"igs. 5, g, and 10, I have given details 

 which were less distinctly seen in the younger discs which I 

 then described. In the preparations represented it is perfectly 

 easy to distinguish two layers of cells in the disc itself, a super- 

 ficial layer of columnar epiblast and a deeper layer of small 

 round cells, which I now regard as mesoblast, between the 

 stellate parablast (which I formerly termed mesoblast) and the 

 epiblast. 



From a re-examination of the discs at an earlier stage, I have 

 come to the conclusion that a thin layer of small mesoblastic 

 cells is present even in the youngest discs, whilst the stellate 

 parablast is certainly developed from wander-cells during their 

 evolution in the pronymph. 



Other groups of cellular elements are also developed during 

 the evolution of the discs. Groups of outlying ovoid cells 

 appear in the parablast around the tracheal vessels of the 

 nymph. These have been described by Van Rees as the 

 mesenchyme (mesoblast?) of the disc. I believe they are also 

 parablastic cells, and that they are subsequently absorbed. 

 Similar groups and strings of ovoid cells also appear in some 

 sections between the stellate parablast and the small- celled 



