1886.] STRUCTCKK OF THK OVUM IN THK DIPNOI. 524 



affected by the borax carmine, and is throughout of a shnilar struc- 

 ture ; there is no structure resembling a nucleus to be seen. This 

 central mass is clearly a further development of the condition 

 described in the second stage. 



In one example, displayed in fig. 23, there is a difference from the 

 condition just described in the presence of a few cells imbedded in 

 the central mass, but clearly distinguishable from it by the nuclei 

 being deeply stained. The cell-protoplasm was, however, hardly 

 distinguishable from the surrounding mass. In the other example 

 (fig. 21) the central mass contained no such cells. 



With the exception that it possesses no nucleus, this structure 

 resembles very closely Platner's figure of the Gasteropod ovum ', which 

 contains a number of cells (" Nahrzellen ") within its substance, 

 derived from similar cells lying around the ovum. 



The first two stages described are about equal in size ; the latter 

 two are also about equal to each other, but considerably larger 

 (twice the size) of the former. The comparison of sizes quite 

 supports my identification of the latter two as the later stages in 

 development ; and there can be no doubt, I think, that they all 

 belong to the same series. 



These structures obviously bear a certain resemblance to the multi- 

 cellular bodies in this fish and in Protopterus ; and if I had not 

 succeeded in finding in Ceratodus another structure undoubtedly 

 corresponding to the multicellular body oi Protopterus, I should have 

 certainly regarded the structures at present under discussion as the 

 representatives of the latter. 



The principal diff'erence ajipears to be the non-formation of any 

 secondary follicle-layer, the absence of any special vascular supply, 

 and the fact that yolk is not formed in the early stages. With 

 regard, however, to the apparent absence of the extra-folhcular layer, 

 it must be remembered that the close resemblance of the layer to a 

 layer of stronia-cells has already been dwelt upon (p. 508). It seems 

 to me very possible that the structure just described is formed by 

 the fusion of the protoplasm of the centrally-placed cells, the nuclei 

 themselves gradually disappearing. 



An examination of better material must, however, settle the 

 question. 



General Conclusions, and Comparison of Ova with those of 

 other Vertebrates and Invertebrates. 

 The general conclusions to be drawn from the facts, in so far as 

 they refer to the Dipnoi, have been partly summed up in the resum6 

 at the end of my former paper on this subject. It may be taken as 

 a proved fact that the ovary in the Dipnoi contains two kinds of 

 structures developed from the germinal epithelium. The first kind 

 is an ovum, the equivalent of a single cell, and is similar in all 

 essentials to the ovum in the Amphibia. The second structure, 

 which is very commonly met with in Protopterus and but rarely in 



' " Zur Biklmig d. Gesclileditsproducten bei den Pulmoiiateu," Arch. f. luitr 

 Aiiat. Bd. xxvi. {\f:M) p. 599. 



