INVEKTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 855 



and merely condensed layer and a distinct membrane. Helix aspersa 

 was always observed to be provided with a true membrane by M. 

 Perez ; it is seen most easily in the earlier stages, when it is easily 

 distended by water; in the mature egg, however, it is difficult to 

 demonstrate its presence, but observations of diiferent ova will pro- 

 bably satisfy the observer as to its existence. 



Turning to the next succeeding phenomena, Perez states that the 

 first sign is a series of changes in the germinal spot, and the sub- 

 sequent appearance of two small nucleoli ; these changes consist in the 

 apj)earance of pale and indistinctly separated granulations, in the 

 midst of which two small nucleoli may be seen ; the germinal vesicle 

 loses its distinct contour and its membrane folds in just like a 

 bladder does when its contents are being expressed ; suddenly it dis- 

 apj)ears and leaves no trace of its existence. 



The clear space thus left is rapidly surrounded by yolk-spheres and 

 two " centres of attraction " dominate over the substance ; in other 

 words, the two nucleoli are developed at the expense of the substance 

 of the germinal spot. As to the connecting " fusiform body," it is 

 supposed that it has nothing in common with a modified nucleus, but 

 that it is due to purely physical causes, which comjDcl the connecting 

 lines of molecules to form arcs between the two nucleolar centres. 

 Experiments by the aid of pressure led the author to the conclusion 

 that the two nucleoli of the germinal spot, rendered free by the disso- 

 lution of the nucleus, become surrounded by rays in conseqiience of 

 the attraction which they exercise on the surrounding protoplasm. 



As to the " polar globule," the author gives the following account, 

 which is of extreme interest. In his view what happens is this: 

 when the nucleoli have attained a certain size, they become provided 

 with a membrane, the presence of which is the cause of the cessation 

 of the movements of molecules around the nucleolus ; the fluidity of 

 the substance which has undergone cohesion increases as the attrac- 

 tive power of the nucleoli disappears ; and the fusiform system tends 

 towards disaggregation ; on the other side there is the vitelline mass, 

 which exerts a certain amount of contraction on the system, which is 

 thereby forced towards the surface of the egg, from which it is finally 

 expressed in a spherical form in consequence of the forces exerted 

 by the molecules of the ifluid. The fusiform radiating substance is 

 expressed in two drops. 



There now remain the two nucleoli (nuclei) without any fusiform 

 connecting mass. They increase greatly in size, and their projier 

 nucleoli are continually subdivided, until at last they form mere 

 granulations; the succeeding stages the author has not been able 

 to observe, but he is not inclined to think that they become fused, 

 but that, rather, one is totally destroyed while the other, persisting, 

 develops two nucleoli, which become the centres of a new system 

 of radially arranged molecules, and which, by aftecting the whole of 

 the vitelline mass, effect its segmentation. He points out that the 

 presence of the two primary nuclei (after the formation of the polar 

 bodies), both of which have had a common origin, is an excellent 

 argument against regarding one of them as being a spermatic nucleus ; 



