MIGRATION OF EMBRYOS. 



57 



have never been fertilised, and this view is supported by the fact 

 that the eggs of unfertilised females among the Nematoda de- 

 generate in the same way without any apparent cause. 



In Entozoa that develop in a short space of time (e.g., Dochmius 

 duodenalis), the early stages are usually passed through while the 

 eggs are traversing the alimentary canal of their host. Occasionally 

 the whole process takes place in the body of the host, especially when 

 they remain there for a considerable period. A longer sojourn in a 

 living host may thus be a necessary preliminary to embryonic 

 development. 



Though our knowledge with regard to the germinal activity of the 

 eggs of Entozoa rests at present upon a comparatively small number 

 of experiments and observations, 1 these are so entirely in harmony, 

 that there is no doubt about the general facts. We can therefore 

 state with confidence that the embryos of oviparous forms develop 

 after the eggs are laid, while those of viviparous (or ovo- viviparous) 

 forms are developed previously, in other words the eggs of all 

 parasites at some time or other, either sooner or later, develop an 

 embryo, 1 * provided that they meet with favourable conditions. 



MIGRATION OF THE YOUNG BROOD. 



The embryos of Entozoa by no means exactly resemble their 

 parents. On the contrary, they never do so, even in the Nematodes, 



FIG. 35. Egg of Distomum 

 hepaticum with embryo. 



FIG. 36. Egg of Both- FIG. 37. Egg of 



riocephalus latus with Echinorhynchus giyas with 

 embryo. embryo. 



1 See the observations of von Willemoes-Suhm, Zcitschr. f. wiss. Zool., Bd. xxiii., 

 1873, p. 343, (Bothriocephalm), and p. 337 (Trematoda). 



2 This holds good also for the generative buds of Gregarines the so - called 

 Pseudonavicellae which, earlier or later (in the body of their host or outside it), develop 

 into embryos. 



