.54 LIFE-HISTORY OF 1'AUASITES. 



of a simple piece of apparatus consisting of a ring of blotting-paper 

 enclosed between two glass slides. By alternately damping and drying 

 the paper, the eggs could be brought into a moister or drier atmo- 

 sphere. The conclusion to which I have been led by the use of this 

 apparatus is, that the eggs of numerous Nematodes (Fig. 32), especially 

 those with a thick shell (Ascaris lumlricoides, A. megalocephala, A. 

 mystax, and many free-living Rhabditidae), are not merely capable of 

 enduring a complete desiccation lasting for weeks and even months, but 

 also alternations between the moist and dry conditions. Development 

 does not proceed, however, save in a damp environment, but it is 

 sufficient that the air be merely moist ; indeed, it has appeared to me 

 that this is actually more favourable than wetting the eggs themselves 

 with water. In damp earth development advances rapidly, but if the 

 earth be dried, development is at once checked, without, however, de- 

 stroying the vitality of the germs. 1 



The same holds good for the embryos ; by desiccation they are 

 rendered quiescent, but resume their vital functions on being moistened, 

 as has been known for some time with respect to those species with 

 free-living young (e.g., Filar ia Medinensis and Khabditis). But all the 

 experiments are not opposed to the general law that a moist environ- 

 ment is necessary for the further development of the egys of Entozoa. Of 

 course this is not the only necessary condition. The degree of this 

 moisture, the nature of the environment in other respects, its 

 chemical composition and temperature, are factors which are of varied 

 importance in different cases. Unfortunately, our knowledge on 

 these points is defective, but one fact may be stated with confidence, 

 and that is, that the eggs of certain Nematodes, especially those 

 having a thick shell like Ascaris, possess an extraordinary power of 

 resistance, and can remain a long time without injury to the 

 development of the embryo 2 even in spirit, turpentine, chromic acid, 

 and various poisonous liquids, fatal to the fully grown worm (Bischoff, 

 Leuckart, Munk). Sometimes the degree of concentration of the 

 liquid has an effect. Vix found that the eggs of Ascaris were de- 

 stroyed by a solution of soap of 0'5 per cent., while in a solution of 

 1 per cent, they continued to develop. Similarly, as I have experi- 

 mentally demonstrated, by means of small holes, artificially dug in 

 the earth and filled with decomposing faeces and urine, the eggs of 

 Ascaris lumbricoides are gradually destroyed ; they are likewise often 

 destroyed through the foulness of the water which surrounds them. 



1 The statement of Davaine (M4m. Soc. Biolog., t. iv., p. 272, 1862), that the eggs 

 of Ascarides inhabiting terrestrial animals undergo development when dried up, rests 

 upon an error. 



2 This is the oase also with the so-called Psoross}>erni*, which aru the germs of Gregari- 

 noid parasites (Cviridium, Lt.). 



