1886.] STRUCTURE OF THK OVUM IN THE DIPNOI. 525 



changes, though the degeneration of a few out of an immense number 

 (see footnote) is hardly proof of this. If I were in a position to deny 

 the presence of a germinal vesicle, the absence of this essential element 

 in the ovum would be evidence of some force in the same direction. 

 On the other hand, the energy showed by the central cells and the 

 cells of the follicular epithelium would be entirely thrown away in 

 this case ; and it is very difficult to imagine the continuance of such 

 a wasteful process in the ovary — the organ chiefly concerned with 

 the preservation of the race'. 



A formation of ova in the Vertebrata by a fusion of cells has been 

 stated to exist, but has been subsequently denied. 



Goette's observations on the formation of the ovum in Bombi- 

 nator^, referred to in my former paper, have been explained away by 

 Nussbaum % who considers that the polynuclear condition described 

 by Goette is the result of the proliferation of the nucleus of a 

 primitive germinal cell, and is not produced by the approximation 

 of the nuclei of a number of distinct cells which subsequently are 

 fused. 



The observations of v. Siebold* are of interest in relation to this 

 question. 



In Apus the ova are formed in spherical acini which contain a number 

 of cells of which one grows at the expense of the rest. This cell 

 finally comes to occupy the whole of the interior of its acinus, the 

 remaining cells dwindling indefinitely ; the nucleus disappears, and 

 yolk-formation sets in. When the cell has been, for the most part, con- 

 verted into yolk it moves down the duct which connects the acinus with 

 one of the branching tubes of the ovary, and there fuses with one or more 

 cells which have been produced in other acini by a similar course of 

 development. The fused mass becomes surrounded by a membrane, 

 and is the ovum. In my preliminary notice in the ' Zoologischer 

 Anzeiger' I have referred to the observations of v. Siebold, and 



differ very widely from a description (Arch, fiir mikr. Anat. 1886) of the 

 degenerating ovum of the trout. 



I may also take this opportunity of referring to an ovum of Ceratodus in a 

 similar condition of degeneration. I erroneously mentioned this ovum (Zool. 

 Anzeig. No. 236) as a nearly fully mature ovum belonging to the multicellular 

 or plasmodial t^ pe ; it may very likely belong to this type, but the appear- 

 ances which it presents are indicative of degeneration, and not of matiu-ation. 

 The follicular epithelium is not separated by any membrane from the contents 

 of the ovum (fig. 2) ; its cells in a few ca&es are loaded with brown pigment, and 

 many of them have migrated into the ovum, the contents of which have been 

 largely absorbed, probably by these cells ; in consequence of this the ovum is 

 collapsed. 



I have noticed a similar state of affairs in the ovary of a newt {Molge waif Hi), 

 which I purpo.se to describe on some future occasion. 



' It has been suggested to me, in conver.sation, by Prof. Lankester that 

 these structures may serve as food for the embryos, being deposited along with 

 the ova, or that they may be reservoirs of nutritive material aiding in the 

 growth of the intra-ovarian ova. Either of thc.«e suggestions appears to ma 

 to be extremely plausible. 



' Arch. f. mikr. Anat. Bd. xviii. 



' Entwickelungsgeschichte der Unke. 



* Beitriige zur Parthenogenese der Arthroptxlen. Leijisic, 1871. 



