98 G. WITENBERG 



of small rodents, provides an instructive example of retardation 

 of embryonic development. The female lays eggs incessantly, but 

 they are not eliminated from the liver and accumulate in its 

 tissue in large macroscopically perceptible masses. As long as the 

 eggs remain in the liver they do not develop. Their development 

 starts only when they are liberated after the death of the host 

 (often via the droppings of the cannibalistic fellow-rodents which 

 devoured the infected animal!) and come in contact with 

 atmospheric oxygen. 



So called 'winter eggs' are practically unknown in helminths; 

 winter interruption of embryonic development occurs in many 

 species, but is caused not by any factor inherent in the egg but 

 by the low temperature only. There is only one indication of 

 winter eggs occurring, namely in the monogenetic trematode, 

 Dactylogyrus vastatot\ living on the gills of the carp, but even 

 this has been questioned recently. Nybelin- and some other 

 authors supposed that this tremiatode lays 'winter eggs' which 

 remain undeveloped throughout the cold season. Groeben^ 

 suggested that this species lays two kinds of eggs, those which 

 develop immediately, and the so called 'Dauereier' which 

 develop only after some time. Bauer and Nikolskaya"* do not 

 accept either of these hypotheses and assert that the cessation of 

 development of the eggs of D. vastator depends on lowered 

 temperature only. 



In many species of parasitic worms, especially in cestodes and 

 trematodes, the newly laid ovum contains a fully developed 

 larva. The larva within the egg may be motionless or it may 

 move freely or rotate. Such a larva either hatches early in 

 natural surroundings entirely by its own efforts, or it remains 

 within the shell in a quiescent state and hatches later in the 

 intestinal tract of the appropriate host. The period of survival 

 of the embryonated egg outside its host varies greatly in 

 different species. Eggs of some species, for instance Heterodera 

 schachtii, a parasite of sugar beet, may hatch not just during one 

 season but over a number of years, even when derived from a 

 single batch^. 



