6i8 THE RESPIRATION AND [pt. iii 



worth while to make further experiments (e.g. submersion in oil) in 

 support of Schwann's conclusions^. 



A good deal of work, however, continued to be done on the 

 effects produced by partial and total varnishing of the egg-shell, 

 and attempts were made to find out just how much of the surface 

 was necessary for satisfactory development. Geoffroy de St Hilaire; 

 Baudrimont & Martin de St Ange; Herholdt; Poselger, and Dareste 

 used varnish, collodion and wax respectively for this purpose, but, 

 as they themselves owned, such eggs lost a good deal of weight 

 during incubation, so that their varnishes must have been permeable 

 to some extent to gases. This explains why in their experiments the 

 allantoic vessels grew quite as usual under the shell. Dareste made 

 a great many observations on these points, but he was not very 

 successful in clearing them up. He stated that, if the part of the shell 

 which covered the air-space was varnished, the allantois would grow 

 over the inner parts of the shell, but not over the membrane separating 

 the egg-contents from the air-space. If the blunt end was varnished 

 before the fifth day, the embryos inevitably died, but if after that 

 time, they did not, for the allantoic vessels had then had time to 

 apply themselves to other parts of the egg's surface. Varnishing the 

 pointed end never had any ill effects. The idea thus grew up (without 

 any real justification) that the air-space had some special significance 

 for embryonic respiration, or at least, that the allantois normally 

 reached the air-space membrane first of all and so made use of its 

 air to a special extent. Dareste also affirmed that, after the varnishing 

 of the obtuse end, the air-space often moved round to the side of 

 the egg. Dusing, who made a thorough examination of the whole 

 subject at Preyer's instigation, was not able to agree with all the con- 

 clusions of Dareste. He used an asphaltic preparation which really 

 was impermeable to gases, and carefully ascertained that the eggs 

 varnished all over with it lost hardly any weight during their 

 3 weeks' incubation. By this means he found that the varnishing of 

 the blunt end of the egg did not lead to a high mortality among the 

 embryos, no matter when it was done, from which he concluded 

 that the allantois does not normally reach the air-space membrane 

 first of all, and that the embryo does not depend upon the air 

 there for the oxygenation of the blood in its vessels. He was able 



1 Asphyxiation of insect embryos while still in their eggs has now become a very im- 

 portant part of economic entomology (see Staniland, Tutin & Wilson) . 



