BY R. J. TILLYARD. 429 



or, as the case may be, diffusion-forces as the motive agent ("rein 

 chemische, resp. DifFusionskrafte als das bewegende Agens 

 anzunehmen"). Lowne postulates for this, and also further on, 

 for the function of the Closed System, at least a temporary open- 

 ing of the thoracic stigmata, which certainly are not wanting in 

 our instance. The difficulty lies here especially in this, that 

 such a function of these stigmata is only concluded ("nur er- 

 schlossen"), but never positively observed. 



On this ground, Ris rejects Lowne's theory on this single 

 last point, and declares the problem not yet solved It seems 

 quite clear that Ris is right, for the following reasons. Sup- 

 posing we grant Lowne's contention concerning the thoracic 

 spiracles, what then'? In the case of aquatic larvae, of what avail 

 would the opening of these spiracles be, seeing that only water, 

 and not air, could be drawn in through them? Or, if it be urged 

 that, at any rate, the young larva might climb out of the water 

 for a short time, and thus fill its tracheal system with air through 

 its temporarily opened spiracles, the answer is very definite, viz., 

 that no such action by the larva takes place, and that the first 

 filling of the trachece with air has been obse7'ved more than once 

 to take place beneath the water. 



Thus the opposition to the Diffusion-Theory comes simply to 

 this, that the main principle of the Theory is not even attacked, 

 but is admitted to be exceedingly probable, were it not for the 

 supposed insuperable difficulty attending the startitig of the pro- 

 cess in the newly-hatched larva. It is well known that the 

 tracheae of the embryo are filled, before hatching, with a pale 

 fluid, generally supposed to be blood-plasma. How, it is asked, 

 can this fluid be withdrawn, and diffusion of air or of oxygen 

 follow at once from the water in the rectum into the tracheal 

 tubes'? The obvious answer to this is, that it cannot be done. 

 And there the matter is allowed to rest, apparently with the 

 overthrow of the Diffusion-Theory. 



Now, I propose, in this paper, to show (1), that a solution of the 

 above difficulty does exist, and (2), that the Diffusion-Theory 

 rests on solid ground, with a good morphological basis for its 

 support. 



