Cua. I. GROWTH OF THE OVARIAN EGG. 481 
the yolk cells present a smaller size and are less crowded, and clearer homoge- 
neous cells mix in considerable numbers with them, so that really this is the 
most fluid part of the egg. From this pomt the yolk increases in density radi- 
atingly, not only toward the periphery, but also in approaching the centre, which 
is not at all to be distinguished from the surrounding part by any sudden dif- 
ferentiation in its constituents.’ 
Although, from quite an early period, the cells around the Purkinjean vesicle 
are smaller than those which are more distant, yet the changes which take place 
in their mesoblasts and entoblasts are identical in all the yolk cells throughout 
the ege. The shape of the egg also gradually changes, with increasing size and 
age, from a flattened, more or less disciform, to a perfectly globular, figure, as it 
becomes more and more detached from the surface of the ovary, against which, in 
the earlier stages, it is very closely pressed. 
Thus we see, that, from the beginning to the maturation of the ovarian ege, 
there is a constant dissimilarity between its two sides, one of which corresponds to 
the position of the Purkinjean vesicle, and the other to the opposite portion of the 
ege, The former contains within and around itself the extreme of albuminous con- 
centration, and the latter the preponderance of oleaginous elements; yet, interme- 
diate between the two sides there is a gradation, both in the proportionate size of 
the cellules and the relative amount of the above-named substances, which unite 
these extremes into one harmonious whole. How far this antagonism is carried 
out in the subsequent phases of the life of the egg will be more fully discussed 
in a future section; but this more we will say here, that, although we have 
had no opportunity for observing the intermediate steps between the maturation 
of the ovarian phase of the egg and the period of slightly advanced segmenta- 
tion, we have still sufficient reason to assume that the same diversified portions 
mentioned above retain the same relative position” during the passage of the egg 
1 See note 1, p. 480. 
2 The clear space, observable in the egg of various 
animals just previous to segmentation, to which the 
name of “embryo cell” has been given, (see Thomp- 
son, 1. ¢., p. 159,) from its supposed intimate connec- 
tion with the formation of the germ, may be identical 
with the white area about the Purkinjean vesicle ob- 
seryed in Testudinata. We would take this opportu- 
nity to express the opinion, that very probably too 
much stress has thus far been laid upon the assump- 
tion that the Purkinjean vesicle performs a peculiar 
and exclusive function in reference to the formation 
of the so-called embryo cells; and, moreover, that 
61 
the Purkinjean vesicle is not to be so definitely sepa- 
rated, as regards its essential elements, from the im- 
mediately juxtaposed substance of similar appearance, 
but should rather be looked upon as the crowning 
point of albuminous concentration, to which the oppo- 
site side of the egg stands in the reyerse extreme of 
a highly oleaginous nature. <A reference to the mode 
of origin of this vesicle shows this conclusively; for it 
is developed as a phase of secondary accession in the 
egg evolution, and not as the primary basis to a suc- 
ceeding structure ever after retaining a significance 
of superior import, and leading, as some would have 
it, to its becoming in the end the essential element in 
