448 On the Intra-ovarian Egg of some Osseous Fishes. [Dec. 9, 



I. The Nucleus and its Changes in the Smaller Ova. 



In the smallest ova the nucleus occupies almost the whole of the 

 interior, and the nucleoli are mostly attached to the inner surface of 

 the nuclear wall. In somewhat larger eggs, the protoplasm sur- 

 rounding the nucleus has increased, and is seen to be divided into a 

 darker internal portion and a lighter external one. The ring of dark 

 protoplasm becomes separated off: from the nucleus in the later stages, 

 and ultimately disappears. The dark protoplasm has no doubt origi- 

 nated from the nucleus. The view that this has been caused by a 

 substance being added from the nucleus is considerably strengthened 

 by an observation made by Ransom, and published in the ' Philosophical 

 Transactions,' 1867. He found in fact that the germinal spots were 

 soluble in some of the constituents of the yolk. At this stage the 

 spots become vacuolate, and assume a variety of different forms, until 

 the nucleus enters a new phase in the development of the intra- 

 ovarian egg described in the following paragraph. 



II. The Larger Ova and the Formation of the Yolk Spherules. 



The egg has almost reached its final size, although far from being 

 mature, when the nucleus is seen to have shrunk a little, and from it 

 project protuberances on all sides. These protuberances or diverti- 

 cula, most of which contain nucleolar particles, are ultimately con- 

 stricted off: from the nucleus, and travel towards the periphery of the 

 egg. A similar transformation of the nuclear contents has recently 

 been observed by Balbiani, Koule, and Fol, in invertebrate ova, and 

 by Will, in amphibia. The buds with their enclosed contents form 

 the yolk spherules, the solid mass in their interior soon breaking up 

 into fine granules. Both Gegenbaur and Balfour speak in support of 

 the view that yolk spherules originate within the egg. As the egg 

 reaches maturity the nucleus degenerates still more, but I believe it 

 never entirely disappears. 



III. The Egg Membranes. 



Much has been written on this subject, and it is still doubtful how 

 many membranes exist. Almost all observers, however, agree that in 

 the mature egg there is a membrane pierced by minute pores, which 

 has generally been called " zona radiata," though other terms, such as 

 vitelliue membrane, egg-capsule, &c, have been applied to it. In the 

 intra-ovarian egg of the gurnard I found a semi-fluid layer inside 

 the zona, corresponding to the " helle Randschicht " described by 

 Gegenbaur in the ova of birds and reptiles. It disappears in the ripe 

 ovum. No membrane external to the zona, such as mentioned by 

 various observers, was seen in this or other fish eggs. 



With regard to the pores in the zona radiata, it seems very probable 



