DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY 



375 



for the formation of a hypodermal thickening in proximity to 

 the point of insertion of the limb, and this thickened part of the 

 hypodermis gives rise, as Ave shall see below (p. 379), to the forma- 

 tion of the whole imaginal thorax, while the hypodermis of the 

 larva disintegrates. 



We must here touch upon the question of the first appearance of the meso- 

 dermal portion of the rudiments of the extremities. In the imaginal discs of 

 developing larvae of Musca, a separation into an outer ectodermal and an inner 

 mesodermal portion is always found. Gaxin (No. 107) derives the mesodermal 

 portion from a differentiation and delamination of the innermost layers of the 



- as 



Fig. 1S5. — Diagrams illustrating the position of the imaginal discs in the larva (.-I) and pupa 

 (B)of Musca (taken from Van Rees). The wing-rudiments are omitted, as, optic disc ; at, 

 antennal rudiment; 61, &'■*, W, rudiments of the three thoracic limbs; bg, ventral chain of 

 ganglia; g, brain; h, so-called "brain-appendage"; m, peripodal membrane; o, aperture of 

 the brain-appendage into the pharynx; oe, oesophagus ; p, so-called "pharynx" ; r, rudiment 

 of the proboscis ; ss, frontal disc ; st, stalk-like connection between the peripodal membrane 

 and the hypodermis ; /, II, III, the three thoracic segments. 



ectodermal portion, and Van Rees has supported this view. Kowalevsky 

 (No. 107), on the contrary, tends to the view that the mesodermal part of the 

 imaginal discs is to he derived from the cells of the embryonic mesoderm. He 

 finds, scattered in the mesoderm, beneath the hypodermis of the larva, so-called 

 wandering cells (Fig. 190 A, vj, p. 383), which differ in appearance from the 

 leucocytes, and which represent the elements from which the formation of the 

 mesodermal part of the imaginal rudiments proceeds. Kowalevsky is disposed 

 to assume an imaginal rudiment for every segment, this rudiment being, 

 however, so delicate and undifferentiated as not to be discoverable in the first 

 stages. From these imaginal rudiments of the mesoderm, the above-mentioned 

 wandering cells would be derived, and would only secondarily become connected 

 with the imaginal discs. 



