ZOOLOGY AND BOTANY, AliOKOSCOPY, ETC. 173 



and disappear in the case of those Nematodes that attach themselves to 

 organs in which it is awanting. A certain amount of moisture, too, is 

 necessary ; and most of the eggs are destroyed by desiccation, and full 

 development is rarely reached in dry conditions. The shell of the egg 

 is not dissolved by the digestive juices, and hatching is not due to the 

 action of their diastases. Hatching does not take place in the stomach 

 but in the small intestine, and particularly in the last third of it. 



The conditions necessary to the liberation of the embryo of Ascarids, 

 Heterakids, and Trichocephalids are three : (1) an alkaline or neutral 

 reaction of the medium : (2) a temperature equal to that of the host ; 

 (3) complete development of the embryo. Liberation is rare at a 

 temperature below 33°. The survival of the ovum depends on the 

 presence of the shell, on its physical and chemical properties, and on 

 the temperature. The shell is composed of chitin, and is therefore 

 resistant to the most diverse chemical agents, and to the digestive juices. 

 At low temperatures the substances dissolved in the medium penetrate 

 so slowly that the shell was long believed to be impermeable, but 

 gases pass through it relatively easily. At a high temperature (33-38°) 

 both dissolved substances and gases penetrate the shell more easily, and 

 the fate of the embryo then depends on the degree of their toxicity. 

 Among the substances studied in the course of Martin's work, chloride of 

 sodium, bicarbonate of sodium, chloride of lime, and chloride of magne- 

 sium are most favourable to the survival of the embryo at a temperature 

 of 33°. Independently of the modifications it produces in the properties 

 of the shell, temperature exercises a direct influence on the embryo 

 itself, and, generally speaking, the higher the temperature the shorter 

 the duration of the life of the embryo. The most complete adaptation 

 is exhibited by viviparous Nematodes, the embryos of which develop 

 at the temperature of the host, within the maternal uterus, during the 

 parasitic life of the worm. In oviparous forms the indifference of the 

 ovum to the chemical nature of the environment and the quasi-imperme- 

 ability of its shell at an ordinary external temperature enable it to 

 survive for a long time, and to develop slowly in water, soil, etc. When 

 ingested by a Vertebrate it is protected by its chitinous shell from the 

 action of the digestive juices, and the embryo emerges in the small 

 intestine, where the necessary conditions of temperature and alkaline 

 reaction are found. Eggs deposited in the intestine do not develop 

 there, because (1) the temperature of the host prevents the development 

 of the embryo in some cases, and destroys it at an early stage in others ; 

 (2) there is an insufficiency of free oxygen ; and (3) the gases arising 

 from intestinal fermentation are rapidly fatal to the embryo. Outside 

 of the body the chief causes of the destruction of the ova are desiccation, 

 the permeability of the shell to toxic gases, the presence of saprophytic 

 fungi, which often attack the shell and destroy the vitellus. 



Fresh-water Mermithidse.* — Eugen von Daday gives an account 

 of the external features, the fine structure of the integument, the 

 longitudinal bands which divide the musculature into bundles and 

 afford insertion to internal organs such as gonads, the muscular system, 



* Math. Nat. Ber. Ungarn., xxvii. (1913) pp. 214-72 (4 pis.). 



April 15th, 1911,. N 



