242 EMBRYOLOGY 



which their organization fits them. They develop into males 

 and females, which copulate and lay fertilized eggs. These 

 develop in the free condition, and a generation of Rhahditis- 

 like Nematodes arises from them. The latter most likely 

 migrate into the larvas of the weevil, and here change into 

 the Allantonema form, described above. Here therefore the 

 process of development is further complicated by embracing 

 two differently formed generations, of which one is free, 

 throughout life ; the other, however, leads in part a parasitic 

 life. This condition, long known as heterogeny, corresponds 

 to the mode of development of llhahditis nigrovenosa, only 

 that in the latter case no such fundamental metamorphosis 

 of the parasitic generation takes place. The hermaphroditic 

 form, ordinarily known as Ascaris nigrovenosa, inhabits the 

 lung of the frog. It produces eggs, the development of 

 which we have described above. The eggs are developed in 

 the parent, from which the embryos emerge in the lung of 

 the frog. From thei^e the embryos pass into the intestine 

 and then out with the fseces, and then develop into males 

 and females, the true Bhahditis form. After copulation, 

 there are developed within the female a small number 

 of young, which abandon its body after they have been 

 nourished by its contents. These young worms likewise 

 exhibit the Rliabditis form, and do not lose it until they 

 have migrated into the lung of the frog, where they are 

 metamorphosed into the hermaphroditic form. The course 

 of development in Bhabdonema strongyloides is, according to 

 the discovery of Leuckart, also similar ; the hermaphroditic 

 form, hitherto known as Ang^dllula intestinalis, inhabits the 

 intestine of man, whereas the dioecious Rliabditis form {Rliab- 

 ditis stercoralis) is found in a free condition. 



Those forms also which, in order to reach their complete 

 development, must live parasitically in two different hosts, 

 show a very high degree of adaptation to a parasitic mode of 

 life. This applies, for example, to Cucnllanus elegans, which 

 is found in the intestine of the perch. The young of this 

 vivipai'ous Nematode pass from the intestine of the host into 

 the water, where they may live free for several weeks, until 

 they meet with some suitable host. This is not the perch, 



