56 LIFE-HISTORY OF PARASITES. 



This is known to be the case especially in the eggs of Nematodes, 

 which were not only hatched, according to Schubart and Eichter, in 

 small aquaria, but also, as already mentioned, in a damp atmosphere 

 and damp earth with even greater certainty. This has also been 

 proved in the case of the eggs of numerous tape-worms (Bothrioceph- 

 alus) and Trematodes. 



In many, almost in the majority of instances, embryonic develop- 

 ment only progresses during the summer months, and in many species 

 only under the influence of a considerable degree of warmth ; thus 

 the eggs of Ascaris lumbricoides require a temperature of 20 C., 

 those of Trichocephalus 22 '5 C., and those of Oxyuris vermicularis 

 as much as 40 C. The eggs of the latter develop a complete 

 embryo in a few hours, and when the temperature is increased, in a 

 still shorter time, T while the eggs of Ascaris and Trichocephalus, which 



differ from those of Oxyuris in being 

 entirely undeveloped at the time that 

 they are laid, require several weeks ; 

 and when the temperature varies, as 

 it generally does in this country in the 

 summer, several months elapse before 

 the young are hatched. Trichocephalus 



FIG. 34. Eggs of Oxyuris ver- * f 



micuiaris ; a, 6, freshly laid ; c, with rarely completes its development with- 

 developed embryo. j n ^ year . ^scaris lumbricoidcs, in the 



natural course of events, requires three or four months, and Ascaris 

 mystax some three weeks. On the other hand, the young of Dochmius 

 duodenalis (especially in warmer climates) are hatched in a few days. 

 Similar variations are found in Trematodes and Cestodes, the eggs 

 being sometimes hatched in a few days (Triccnophorus), at other times 

 requiring weeks (Ligula) or even months (Bothriocephalus latus, Dis- 

 tomum hepaticum, &c.) for their full development. This, however, only 

 applies to the summer months ; in winter, even in a heated chamber, 

 development goes on slowly and irregularly; in Ascaris mystax, for 

 example, the first traces of cleavage appear only after several months. 

 Besides temperature, other circumstances are of considerable 

 importance. There are individual differences between eggs them- 

 selves ; embryos rarely develop in them simultaneously ; one egg may 

 have hardly commenced to divide, while another contains a fully 

 formed embryo. Numerous eggs also, under conditions favourable in 

 other respects, never develop, but undergo a process of degeneration 

 in which the whole mass becomes granular and semi-transparent, and 

 all the details of its structure vanish. It may be that these eggs 



1 In sunshine Vix saw an active embryo develop in a quarter of an hour in the eggs 

 of Oxyuris. Zcitschr. f. Psychiatric, Bd. xvii., p. 65, 1860. 



