CONDITIONS OF DEVELOPMENT. 55 



All that these experiments show is that there is a limit to the power 

 of resistance possessed by the eggs of Nematodes. 



All the cases just cited, however, by no means lead us to infer 

 that power of resistance is not shown by the eggs of other Helminths, 

 though certainly they do not show it to so great an extent as the 

 Nematoda ; but, compared with other animals, unfavourable conditions 

 of environment take a much longer time to destroy the eggs, and 

 this is no doubt owing rather to the simple fact that the shells of the 

 eggs of these parasites are unusually thick, than to any peculiarity in 

 their protoplasm. In this connection it is important to notice that the 

 eggs of Helminths are not only usually provided with a thick firm shell, 

 but frequently possess in addition a simple or more complex accessory 

 covering of some kind, which occasionally gives them a remarkable 

 and characteristic appearance. This additional protective covering, 

 besides serving to increase their power of resistance, often has other 

 functions ; for example, the eggs of Pentastomum tcenioides, which 

 inhabits the nasal cavity of dogs, have a folded outer layer which 

 enables them to adhere to various bodies when they are ejected from 

 the nose of their host. In a similar way the various filamentous or 

 tufted prolongations of the outer egg-shell 

 (Fig. 33), or the coating of albumen which is 

 sometimes to be found (Fig. 32 a) on the egg- 

 shell proper, serve to secure the attachment 

 of the egg to any body with which it comes 

 in contact. The eggs of Tcenia frequently 

 leave the body of their host enclosed in a FIG. 33. Egg of a tape-worm 

 living covering the proglottis- which pos- *<>** 

 sesses a certain capability of locomotion, and therefore aids consider- 

 ably in the dispersion of the contained ova, which are thus rendered 

 more independent of external agents. In spite of all these arrange- 

 ments, thousands of the eggs of Helminths are destroyed by the 

 unsuitableness of the environment; but this is of no importance, 

 considering their immense fertility. . 



Assuming that the eggs attain to favourable conditions, let us now 

 trace out the further course of their development. In the first place 

 it must be remembered that the eggs reach the exterior in very 

 different stages of development ; in many instances (e.g., Acantho- 

 cephala, Tcenice, many Distomidse, &c.) the embryo is already formed ; 

 in others, again, the egg contains merely the original cell. The presence 

 of an embryo, however, is the preliminary condition of any further 

 change. The eggs that, when extruded from the body of their 

 host, are either not at all or only incompletely developed, at once 

 undergo the process of forming the embryo, and the young is hatched, 



