637 



the follicular layer. In a later stage the ovum consists of a mass of 

 yolk particles among which are scattered cells some of which are the 

 remainder of the central cells and others have migrated in from the 

 follicular layer. The formation of the yolk during this stage is still 

 actively going on in the follicular layer. In later stages the activity of 

 the follicular cells has ceased and they no longer contain yolk par- 

 ticles, though they are still somewhat large in comparison to the folli- 

 cular cells of the other kind of ova ; a delicate membrane divides the 

 follicular layer from the ovum ; the latter contains a very few cells in 

 its interior, and these soon altogether disappear and the ovum is fully 

 mature. I have not been able to detect any traces of a germinal vesicle. 



At no period in the development of the ovum is it possible to 

 recognize any single cell within the follicle, which is conspicuously 

 larger than the rest, or different in any way ; nor is there any break in 

 the continuity of the follicular layers ; the whole contents of the fol- 

 licle ultimately become the ovum, and it seems impossible therefore 

 to avoid the conclusion that the ovum has the value of more than a 

 single cell. I am not aware of any Vertebrate in which a similar mode 

 of Oogenesis has been stated to occur. Goette's description of the 

 origin of the ovum in Bomhinator by a fusion of cells has been con- 

 tradicted by Nussbaum (Arch. Micr. Anat. 18. Bd.) and others. In Apus 

 v. Siebold has described and figured the origin of the ovum from a 

 fusion of two or three ova but there has been no confirmation or con- 

 tradiction of his discovery. The consensus of opinion appears to be 

 that throughout the animal kingdom the ovum represents a single cell. 

 In considering the ovum it appears to me necessary to bear in mind 

 the homology that exists between the ovum and its ancestor the Pro- 

 tozoon, to which Lankester (Notes on Embryology and Classification 

 p. 7) has directed attention. Physiologically as well as structurally 

 there is a close correspondence between the two, and I would suggest 

 that the fusion of cells in the Myxomycete and Protomyxa to form 

 the adult is perhaps recapitulated in the ovum of Lepidosiren. 



2. Ceratodus. In Ceratodus as in Lepidosiren there are two 

 kinds of ova ; but the second kind of ova, those formed by the fusion 

 of a number of cells, is very much rarer in the former, than in the lat- 

 ter type. Out of many hundreds of sections I only succeeded in disco- 

 vering two unmistakeably of this kind; one was in the earliest stage 

 found in Lepidosiren \ a central mass of cells surrounded by a folli- 

 cular layer, vascular sheath and secondary follicular layer; the second 

 ovum was almost mature but the follicular cells not yet separated by a 

 membrane from the ovum. 



London, Oct. 5th i886. 



