290 ANIMAL PARASITES. 



soon see, must be to a certain extent capable of a migration also 

 in the mature state. 



The migration in the young state consists in the escape out- 

 wards of the eggs of these worms with the human faeces, with 

 which they get into dungpits, cesspools, and sewers, as Puchter 

 and Von Siebold have already stated. Here they are surrounded 

 with water, and have to bear the most various temperatures, but 

 are transferred with manure and by the rain to the fields, 

 meadows, and drinking-water. The tolerably hard shells of the 

 eggs protect the germs contained in them with the yelk from all 

 external injurious influences, and permit the latter, as appeared 

 from the investigations of Newport and BischofF, and has now been 

 ascertained by Verloren and Richter, to become further developed, 

 and allow of the completion of the process of segmentation out 

 of the uterus of the worm, or they envelope the embryo which is 

 ready formed in the uterus of its mother on its going forth into 

 the external world. The latter, however, appears to be of rarer 

 occurrence in man, and we may, perhaps, admit that the 

 nematode worms occurring in the human subject rarely produce 

 and harbour the young in their uteri, which already resemble 

 their parents in external form, although they are only of a 

 microscopic size. This probably only occurs in the Filaria ; the 

 other species are mostly oviparous. I will here treat at once 

 of the process of the formation and development of the eggs and 

 seminal corpuscles in the nematode worms. 



With regard to the formation of the eggs of the nematode 

 worms in the ovigerous tubes, particular credit is due to Bagge, 

 Kolliker, and, above all, to Meissner ; with regard to the formation 

 of the seminal corpuscles, to Nelson and Meissner; for the obser- 

 vation of the entrance of the latter into the vitellus, whether 

 through an actual micropyle of the vitelline membrane, or in 

 consequence of a mechanical pressure into the egg whilst still 

 destitute of a vitelline membrane, and the changes of the vitellus 

 which afterwards take place (which had been ascertained by 

 Barry in the mammalian ovum, and by Newport in that of the 

 amphibia), to Nelson, who has been violently attacked in various 

 ways, but with exactly the same injustice as Keber ; and, after 

 him, especially to Meissner, and Thompson of Glasgow, and also 

 to Bischoff and Leuckart ; and with regard to experimental 

 investigations on the eggs of nematode worms in water and other 

 fluids, to Verloren, and, independently of him, to H. E. Richter. 



