Cuap. I. DEVELOPMENT OF THE YOLK. 459 
which, at a certain age, runs in a plane passing through one of its great cir 
cles (Pl. 8, fig. 3, and 3a). 
eggs can be rolled backward and forward, by tipping the watch-glass which holds 
After carefully dissecting away the envelopes, the 
them, or by blowing upon the serum in which they swim, so as to disturb its 
level; and as the several eggs pass by the eye, one is reminded of glass globes 
whirling along, freighted on one side with golden pebbles. The splendor of such 
an apparently trifling object needs to be seen to be appreciated. Now follows 
the encroachment of the granulated portion of the egg upon the clear space, at 
This 
proceeds after various modes: sometimes the clear space fills wp in a gradual way, 
as if a deposit was forming in it, (Pl. 8, fig. 6, 6a,) till the whole spherical cavy- 
the time the egg has reached the size of about ;4, of an inch in diameter. 
ity is occupied by a dense, equally distributed, coarsely granular mass; at other 
times a projection starts out from the darker part into the clear fluid, (PL 8, fig. 
10,) and divides it into halves, each of which gradually darkens with deposits ; 
again, the granules appear, as it were suddenly, in heaps, (Pl. 8, fig. 9, 9a, 13a, 
15,) throughout the homogeneous medium, and, gradually extending their arms to 
each other, anastomoze, and inclose clear hyaline spaces, (PI. 8, fig. 15,) the so- 
called oil drops! (Pl. 8, fig. 7, 16a, 16b); finally, a ring of coarser materials 
appears near the centre, giving the egg a zonated appearance, (Pl. 8, fig. 18a, 19,) 
with the germinal vesicle at times between the dark bands. This last phase is 
found in eggs of about one twentieth of an inch in diameter, which are easily 
In fact, 
the different features mentioned above correspond more or less to a certain stage 
seen with the naked eye; and we believe it to be peculiar to this age. 
of the development of the yolk cells. Beyond the last stage, just mentioned, we 
cannot trace this progressive growth step by step, on account of the opacity and 
size of the eggs; but thus far it is perfectly reliable that these diverse appearances 
are normal, since they were recognized, not in one only, but in as many eggs as 
were examined. By this time the egg has assumed a uniform bright yellow color 
on that side where the germinal vesicle shines through as a clear (Pl. 8, fig. 17, 
17a) but dark spot, immediately surrounded by a very light yellow ring, which 
shades off into the deeper color beyond. As the egg progresses toward its full 
development, the yellow color of the yolk grows deeper, (PI. 9, fig. 4, 5, 6, 7, 8, 
1 These clear spaces must not be confounded with 
those which are observed in the spheres of segmenta- 
tion, though they originate probably in the same man- 
ner. That there are eggs containing oil drops, cannot 
be doubted; they are frequent in the eggs of Fishes, 
but they differ greatly from the albuminous clear 
spaces of the Turtle’s eggs just described. Much 
more extensive investigations upon the structure of 
the eggs of animals of different classes, in their suc- 
cessive stages of development, are still required, be- 
fore satisfactory comparisons can be instituted between 
them and the features peculiar to different types 
pointed out. Comparative Ovology is a branch of 
Embryology yet to be founded. 
