BY R. .T. TII.r.YARD 405 



tlie failure of tlif larva to emerge from the pronymphal sheath 

 was correlated with its failure to bring this strong iuini})ing 

 apparatus into action, to aid in the required distension of the 

 head. 



My one remaining hope of discovering something about this 

 important organ lay in a study of sections of a pronymph. As 

 already stated, the pronymph studied on p. 394 was remo\cd in 

 a moribund condition, after it had lived for three and a half 

 hours, and was fixed in hot water. Though it was almost dead 

 when the hot water was poured upon it, the sudden contact with 

 this li(|uid caused it to burst the pronymphal sheath, and flow 

 c<jmpletely out of it. Thus the animal that T had fixed was, in 

 a sense, the true larva, but irlth the form af the proni/mph; for 

 its eyes and head did not expand t(^ the true larval form, its 

 labium and legs remained directed backwards as in the embryo, 

 and its abdomen was still of the slender form seen in the pro- 

 nymph. This little animal was carefully double-embedded in 

 celloidin and paraffin, and cut into sagittal sections of 7/a thick- 

 ness. One of these, a little to the right of the median sagittal 

 plane, is shown in Text-fig. 2. 



In order to understand this section, I have also shown, in 

 Text-fig. 3, a drawing which T made, by means of the camera 

 lucida, of the anterior portion of the same pronymph, at about 

 two hours old. It must be remembered that the cephalic heart 

 had ceased to beat almost directly after the pronymph had 

 hatched. Thus we cannot see the shape and position of that 

 organ in the drawing; and if we want to search for it in the 

 sections, we must bear in mind that it had either collapsed, or 

 assumed some stable form pri(jr to the period of fixation. Text- 

 fi<j;.3 shows us the exact course of the blood-circulation in the 

 head-region during the pronymphal stage, and also the peculiar 

 position of the midgut, pushed forward into the thorax, with the 

 oesophageal valve and gizzard sunk deeply into it, and the crop 

 projecting in front of it, just below the end of the dorsal aorta. 



Now, if we study the series of sections, we at once notice the 

 very remarkable fact that, at the time of fi.\:ation, ///'' (infrrior 



