18 
EMBRYONIC DEVELOPMENT 
extremes may succeed each other in both directions. The axis of rotation is chan 21x10- 
as the form of the vitelline sphere itself temporarily changes, under the incessant 
contractions of the embryonic mass. 
The direction, or movement of revolution is sometimes from right to left ; in others, 
from left to right, in an horizontal plane. In some others again, that movement is 
either forwards or backwards in a vertical plane.* The embryo, therefore, is abso- 
lutely free, and can assume any position and turn in any direction. For, the direction 
itself may change almost instantly, that is to say, if an embryo moves now from 
right to left, the next moment it may move from left to right. I have mpeatedly 
seen a vitelline sphere, artificially pressed out of its external envelope, continue to 
move as if still enclosed. When crushed to the isolation of the constituent cells, all 
of these, large and small, began to dance, turn, whirl independently of each other, 
as if controlled by an irresistible impulse of their own. 
Another phenomenon which illustrates the independence of the cells, took place 
under my eyes. An embryonic sphere, while in motion, lost about one third of its sub- 
stance, the cells of which became loose within the albuminous zone, and were carried 
by the current, around the remaining portion of the original vitelline sphere. These 
loose cells, after a while, congregated into a small sphere, and for a moment one would 
have thought that there were originally two embryos in the same egg, a large and 
a small one. Soon, however, the small sphere was destroyed by the other and its 
cells again loosened, when it rebuilded itself once more. But now it became a per- 
manent little sphere, which, in its motion within the same area as the large one, 
caused many irregularities and perturbations, in the general movements of the latter. 
We have here one of the most attractive and wmnderful phenomenon which nature 
may display ; a phenomenon hidden from the eye of the great multitude of men. How 
many are there, who, at a glance into that sanctuary, would feel powerfully impressed 
with the thought that a divine mind, an allwise Creator is the author of the small as 
well as of the great in the whole universe. A moment of simple reflection would 
convince them that such an activity like that which they thus behold, in a little 
sphere of organic substance, cannot be accounted for by the mere attributes of mate- 
rial forces. 
§ 3. About twelve hours after the motion has begun, the embryonic mass, still 
circular in general form, assumes now two aspects in its substance: 1st. The central 
part, which is composed of large cells either transparent or semi-transparent, contain- 
ing a milky fluid at divers degrees of development (fig. 55, a . Ac.); and 2d, the 
peripheric layer, composed of ordinary vitelline cells from the surface of which vibrillse 
now will grow (fig. 56). 
§ 4. The embryonic mass, after a lapse of another twelve hours, acquires a plastic 
*The horizontal and vertical planes are here spoken of with reference to the observer's orientation. 
