1886.] STRUCTURE OF THK OVUM IN THE DIPNOI. 509 



it possible that the greater part of the tunic, which has apparently 

 the structure of ordinary connective tissue, is iu reality derived from 

 the original germinal epithelium, a view which tallies with the fact 

 that in some instances the cells of the tunic appear as if about to 

 assist in forming the follicular epithelium of some of the developing 

 ova." 



The nuclei of the peripheral layer of cells are much like those of 

 the proliferating germinal epithelium, though not quite so darkly 

 stained — perhaps for mechanical reasons. Their shape varies con- 

 siderably, some being rounded and some more fusiform ; but these 

 two extremes are united by numerous intermediate conditions. For 

 the most part the nuclei have taken up the staiuing-fluid unequally ; 

 a patch at one extreme of the nucleus is more darkly coloured, and from 

 this radiate slender threads towards the opposite extreme of the 

 nucleus ; the interstitial part of the nucleus is stained of a pale pink. 

 The different forms of these nuclei are represented in fig. 5. The 

 differential staining of the nucleus undoubtedly corresponds to the 

 differentiation of its substance ; and the star-like form of the darkly 

 staining part suggests a connection with the phenomena of nuclear 

 division ; but I have not observed any cases in which the centres of 

 the star-like bodies in two adjacent nuclei were opposed ; in every 

 instance the darkly stained extremity of the nucleus was directed 

 away from the germinal epithelium and along the axis of the layer 

 of cells ; this uniformity in the nuclei gives them the appearance of 

 being in rapid motion, of being as it were swept along by a current 

 round the periphery of the sphere of cells. 



Here and there the continuity of the layer of cells is interrupted 

 by blood-vessels {b.l), usually of small dimensions, which are the fore- 

 runners of the richly developed vascular suppl}-^ of these same bodies 

 in later stages of development. The appearance of blood-vessels was 

 more common on that side furthest from the germinal layer than on 

 that nearest to it. 



Towards the opposite extremity of the sphere of cells, i. e. that 

 furthest removed from the outside of the ovary, the character of the 

 nuclei of the peripheral layer of cells becomes changed. In this 

 region the nuclei have lost the peculiar arrangement of the nuclear 

 substance and present the appearance of ordinary nuclei (fig. 5 a) ; 

 that is to say, they are oval bodies with finely granular contents and 

 here and there round, darkly stained particles which are disseminated 

 throughout the nucleus. 



Within the peripheral layer of cells is another layer of cells which 

 becomes ultimately comparable to a follicular epithelium, Even iu 

 this early stage of development it is for the most part distinguishable 

 from the outer layer of cells ; the distinction is not only in the 

 character of the cells and their nuclei, but in an absolute line of 

 demarcation which separates the two layers ; this consists (fig. I, x) 

 of a narrow band of structureless substance, which bears the closest 

 possible resemblance to a substance produced by the fusion of some 

 of the central cells of the sphere, and which will be described shortly. 

 It is possibly formed by a metamorphosis of the peripheral layer of 



