1886. ] STRUCTURE OF THE OVUM IN THE DIPNOI. 509 
it possible that the greater part of the tunic, which has apparently 
the structure of ordinary connective tissue, is in 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 staining-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 ceatres 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 (4.2), usually of small dimensions, which are the fore- 
runners of the richly developed vascular supply 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, ¢. 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. 5a) ; 
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 in 
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. 1, 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 
