6 10 THE STRUCTURE AND DEVELOPMENT 



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

 In 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 inde- 

 pendently of the nests. 



In support of my 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. Tubularia 

 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 indistinct : 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 Weismann 1 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) develops into an 

 ovum, 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 

 entirely converted, by a series of remarkable changes, into 

 nutritive reservoirs. 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 modified cells of the germinal epithelium. 

 In Elasmobranchii their very early appearance, and the difficulty 



1 Zeit. fiir wiss. Zool. Bd. xxvn. 



