and Fertilization in the Nematoidca. 181 



favour of its presence. Supposing, therefore, that Auerbach's 

 opinion be correct, Thompson's observation would no longer 

 tend to sujjport Nelson, but would rather speak unexpectedly 

 in favour of Meissner. 



A membrane is a thin layer of a substance, the chemical or 

 physical pro])erties of which (tenacity, firmness, density, &c.) 

 are sliarj)ly distinguished I'rom those of the media in contact 

 with it on both sides. The periphery of an Amoeba is undoubt- 

 edly formed by a layer of greater density than the rest of the 

 parenehynm of the body. But it seems not improbable that 

 the parenchyma increases gradually in density from within out- 

 wards, and that the external denser layer is not clearly defined 

 towards the inner, softer parenchyma. In this case no true 

 membrane is present. We can only speak of a denser layer or 

 region. Mohl has already noticed this condition in plants, and 

 distinguished every dense outer layer, which cannot be clearly 

 separated from the inner substance, by the name oi pellicula, in 

 opposition to the idea of a membrane. The term pellicula is 

 perhaps not very happily chosen. 



In our opinion, the eggs of Ascaris mystax and A. suilla are 

 in this respect in the same condition as the Amoeba. We have 

 to distinguish two ditferent things in the yelk, — in the first 

 j)lace the vitelline granules, and then a transparent, colourless, 

 uniting, intermediate substance. The outer layer of the eggs is 

 formed only of the latter ; no viteUine granules are contained 

 in it. Of this layer Meissner has made his vitelline membrane. 

 It is, however, as already stated, not a membrane, but only the 

 intermediate substance, becoming more and more dense exter- 

 nally. It is just because this intermediate substance has a 

 greater density at the periphery, that the vitelline granules do 

 not penetrate into the outer layer. 



All observers agree that the eggs are surrounded by a mem- 

 brane in the lower part of the tube. This membrane is produced 

 by the outer granule-less layer of the yelk becoming sharply 

 defined in opposition to the interior of the egg. But where the 

 differentiation commences, it is difficult to say. Tor this reason 

 Bischotf may, to a certain extent, be right in asserting, that the 

 eggs in the vitellogene are not surrounded by a membrane, as 

 the membrane is not yet perceptibly ditferentiated from the yelk. 

 On the other hand, Meissner is not quite in the wrong in main- 

 taining the presence of the membrane, as it is already in course 

 of formation. Whilst, according to Meissner, the vitelline 

 granules are formed in iiis supposed germ-cells. Nelson and 

 Bischotf, on the contrary, see tlie place of formation of these 

 granules in the granular longitudinal processes of the vitello- 

 gene. Thompson, who supposes that the deposition of the vitel- 



