30 ILLINOIS BIOLOGICAL MONOGRAPHS [150 



The hypoderm is derived directly from that of the larva. In the larva 

 it is a very thin layer lying immediately beneath the cuticula and having 

 only very few nuclei except in the rudiment of the nerve cord (Figs. 21, 49). 

 Figures 14 and 20 are somewhat misleading in this respect as it was im- 

 possible to determine the division line between the hypoderm and the 

 underlying muscles. Even in the young parasitic forms the hypoderm is 

 still comparatively thin (Figs. 15, 51, 65, 66). 



The nerve cord arises as a thickening in the hypoderm (Figs. 15, 55, 58) 

 along the ventral side as indicated by the two rows of nuclei in the larva 

 (Figs. 14, 20, 56) and may therefore be regarded as derived directly from 

 the nerve rudiments of the latter. 



The derivation of the brain is more difl&cult to trace and an exact 

 determination will have to be postponed untU the larva can be studied 

 more thoroly. It arises in the posterior end of the proboscis rather late in 

 development. In the five day parasite (Figs. 50-53) and also in the six 

 day form (Fig. 11) its location is not yet definitely indicated. In the nine 

 day form (Fig. 15) it is indicated by a slight enlargement of a ring of cells 

 around the proboscis just in front of the division between the proboscis and 

 body. In the 12 day stage the cells have become enormously enlarged, 

 are located just outside of the muscle strands of the larval proboscis (Fig. 57) 

 and remain connected with the hjqpoderm only at the extreme anterior end 

 and on the ventral side (Figs. 67-69). 



Entodermal derivatives. The young larval parasite possesses no entoder- 

 mal derivative except the intestine and this develops directly from that of the 

 larva. Its development is at first very rapid, so that in the five and six 

 day stages (Figs. 11, 50-52) it makes up a large part of the bulk of the para- 

 site and even in the nine and twelve day stages (Figs. 15, 66) it is relative- 

 ly enormous. 



Mesodermal derivatives. On account of the minuteness of all the cells 

 and the indefinite staining reactions it has been impossible to connect 

 the mesodermal derivatives definitely with larval structures. It is possible, 

 however, to outline their appearance in the early parasitic stages. 



In the five and seven day stages the muscles appear as minute cells 

 between the intestine and the hypoderm (Figs. 50, 51, 54). In the nine day 

 stage they are clearly outlined as a continuous layer of elongated cells lying 

 just inside of the hypoderm (Fig. 55). 



Since the parenchyma appears very late its origin will be taken up in 

 the discussion of the later development. 



The reproductive organs appear in the five day stage as a double row of 

 cells on each side of the intestine, slightly dorsal in position, along the main 

 part of the body (Figs. 50, 51). In the nine day stage (Fig. 55) and more 

 clearly in the twelve day stage (Fig. 66) they appear as two definite ridges, 

 just inside of the muscle layer, dorso-lateral to the intestine, and extend 

 almost the entire length of the body. 



