400 BULLETIN OF THE 



the yolk, and he evidently considers the disappearance of the germina- 

 tive vesicle * as a criterion of that event. 



Besides the entire absence of the germinative vesicle, not the least 

 trace of which could by any means be made visible, this stage is charac- 

 terized by a temporary recession of the yolk granules from the periphery 

 of the egg, during which the vitelline membrane is gradually formed, 

 ])robably by a condensation of a superfical layer of the protoplasm. The 

 return of the granules to the periphery is immediately followed by the 

 contraction of the whole yolk and the contemporaneous exudation of a 

 quantity of clear liquor ovi. Thus is completed the formation of the first 

 cleavage sphere, which is throughout homogeneous. 



As regards the formation of a nucleus in this first segmentation sphere, 

 Auerbach rejected the idea that it appears either as an entirely new 

 centrally located structure, or as a metamorphosed persisting germinative 



* In the eggs of both these nematodes there is a slight deviation from the tmly 

 oval outline ; one end is slightly more obtuse than the other. The more pointed end 

 is characterized, according to Auerbach, by several other peculiarities. It is the one 

 which is in advance as the egg passes through the oviduct, and therefore that which 

 is first exposed to the spermatozoa, probably also the part into which the spermatozoa 

 penetrate. Perhaps this accounts for certain advantages which the narrow seems to 

 possess over the more obtuse end. It is at the former that the polar globules are 

 found ; of the two spheres which result from the first segmentation the more volumi- 

 nous occupies the narrower end ; in its changes this anterior sphere slightly anticipates 

 the hinder one, and is subject to fewer variations from the norm ; and finally, it is 

 this portion of the egg from which the anterior part of the worm is produced. 



Unfortunately, Auerbach has given no account of the place and manner in which 

 the polar globules are eliminated from the yolk. If their constant appearance at the 

 smaller pole of the egg could be taken as evidence that they were eliminated at that 

 j)ole, — an assumption which has a certain amount of support in the less granular 

 condition of that end of the yolk (see Fig. 27, loc. cit.), and in the fact that both pro- 

 nuclei arise at the poles of the vitellus, — then the observation would command par- 

 ticular attention as showing that the almost universal relation of polar globule and first 

 cleavage plane is not in this case maintained. 



The appearance of the pronuclei at opposite poles of the egg is not easily reconciled 

 with Auerbach's ideas, for the female pronucleus, we must now assume, makes its ap- 

 pearance near the point where the polar globule arises, and the male pronucleus at 

 the large end of the Q^g could hardly have arisen from the influence of a spermatozoon 

 penetrating at the smaller. In view of the fact that Butschli ('75, pp. 203, 204) finds 

 the polar globule in non-parasitic nematodes usually at the equator, though sometimes 

 nearer the vaginal pole, and that he has observed its transportation from the place of 

 its origin to the smaller (vaginal) pole, it is perhaps safe to infer that Ascaris and 

 Strongylus offer no exception to the rule that the polar globule makes its appearance 

 in the plane of the future first segmentation, in which event a sub-polar position of 

 the globule would probably correspond to the cases of oblique segmentation so fre- 

 quently observed in the nematodes. 



