242 EMBRYOLOGY 



w'hicli their organization fits them. Thej 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 larvee of the weevil, and here change into 

 the Allantonema form described above. Here therefore the 

 process of development is farther complicated by embi^acing 

 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 there the embryos pass into the intestine 

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

 and females, the true lihabditis 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 lihabditis form, and do not lose it until they 

 have migrated into the lung of the fi'og, where they ai-e 

 metamorphosed into the hermaphroditic form. The course 

 of development in Rhabdonema strongyloides is, according to 

 the discovery of Leuckart, also similar ; the hermaphroditic 

 form, hitherto known as Anguilhda intestinalis, inhabits the 

 intestine of man, whereas the dioecious ]ihabditis form (lihab- 

 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 Cncullanus elcga^is, which 

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

 vivipai'uus 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, 



