STllUCTURK AND DEVELOPMENT OF VERTEBRATE OVARY. 435 



than these nests, must be regarded as giving origin to the ova, 

 and the nests should be looked on, in my opinion, as connected 

 more with the nutrition than with the origin of the ova. Iti 

 favour of this view is the fact that as a rule comparatively few ova 

 are developed from the many nuclei of a nest ; while against the 

 comparison with the egg tubes of the Invertebrata it is to be borne 

 in mind that many ova appear to develop independently of the nests. 



In support of ray view about the nests there may be cited 

 many analogous instances from the Invertebrata. In none of 

 them, however, are the phenomena exactly identical with those in 

 Vertebrata. In the ovary of many Hydrozoa {e.g. Tuhulana 

 mesembryanthemum^ out of a large number of ova which develop up 

 to a certain point, a comparatively very small number survive, and 

 these regularly feed upon the other ova. During this process 

 the boundary between a large ovum and the smaller ova is indis- 

 tinct : in the outermost layer of a large ovum a number of small 

 ova are embedded, the outlines of the majority of which have 

 become obscure, although they can still be distinguished. 

 Just beyond the edge of a large ovum the small ova have begun 

 to undergo retrogressive changes ; while at a little distance from 

 the ovum they are quite normal. An analogous phenomenon has 

 been very fully described by Weismanu^ in the case of Leptodera, 

 the ovary of which consists of a germogene, in which the ova 

 develop in groups of four. Each group of four occupies a 

 separate chamber of the ovary, but in summer only one of the 

 four eggs (the third from the germogene) developes into anovutn, 

 the other three are used as pabulum. In the case of the winter 

 eggs the process is carried still further, in that the contents of the 

 alternate chambers, instead of developing into ova, are entinly 

 converted, by a series of remarkable changes, into nutritive reser- 

 voirs. Fundamentally similar occurrences to the above are also 

 well known in Insects. Phenomena of this nature are obviously 

 in no way opposed to the view of the ovum being a single cell. 



With reference to the origin of the primitive ova, it appears 

 to me that their mode of development in Mammals proves beyond 

 a doubt that they are moditied cells of the germinal epitlielium. 

 In Elasmobranchii their very early appearance, and the difficulty 

 of finding transitional forms between them and ordinary cells of 

 the germinal epithelium, caused me at one time to seek (unsuccess- 

 fully) for a diifereut origin for them. Any such attempts appear 

 to me, however, out of the question in the case of Mammals. 



The egg membranes. — The homologies of the egg membranes 



in the Vertebrata are still involved in some obscurity. 



In Elasmobranchii there are undoubtedly two membranes 



present. (1) An outer and first formed membrane — the albu- 



' ' Zeit. fiir wiss., Zool,' Bd., xxvii. 



