ANIMALCULE ALLIED TO NOTOMMATA. 
339 
arithmetical progression, though this is not improbable, since in an early ovum I have 
once observed the separation into two distinct portions, in each of which there wero 
several large granules or oil-globules, Plate XXXIV. fig. 5". 
Each granular point, however, seems subsequently to become the nucleus of a cell, 
round which the wall is developed. After a short time the numerous cells assume a 
more definite arrangement (Plate XXXIV. fig. 4), and are grouped together in masses, 
at which time, from the slight irregularities in their contour, the fine chorion is seen 
around them, forming an envelope for the whole. 
By and by the groups of cells become more distinct masses, and a larger collection 
of them at one end of the ovum (Plate XXXIV. fig. 5) indicates the future position of 
the head ; while smaller subdivisions are symmetrically arranged around another 
group, which finally becomes the stomach. 
Soon after this period, the cells, or groups of them, evidently become developed 
into tissues, and the embryo begins to assume a definite shape, and the outline of the 
tegumentary covering is visible. Presently a slight ciliary movement is observed at 
one end, indicating the head of the animal, and the chorion shortly after bursting, 
the embryo becomes free, although closely surrounded by the delicate contractile 
membrane of the ovisac. 
Having arrived at this epoch (Plate XXXIV. fig. 10 A), the perfection of the organs 
rapidly takes place, and we begin to recognise the stomach, salivary glands, ovary, and 
the ribbon-shaped muscles of voluntary motion ; in these latter the original nucleus 
of the cell, which has elongated and been developed into muscular tissue, is still 
plainly visible, imbedded, as it were, in the fibre. The embryo has now voluntary 
motion, for while rapid ciliary movements are constantly going on around the head, 
the animal frequently contracts and extends its body, rolls over, or changes its posi- 
tion in the ovisac. The jaws may now be seen, and are not unfrequently moved as 
if exercising or trying their power. The red eye has previously been visible, and 
now and then the head of the animal is so placed that we are enabled to look down 
upon it, and observe the symmetrical position of the jaws with the eye placed oppo- 
site to the entrance of the mouth and pharynx, a position it is difficult to meet with 
for any continued period in the adult, from its instability and frequent movements 
under the glass. 
The embryo may now be said to be ready to quit the parent, but some time pre- 
vious to its exit it is seen exercising various and energetic movements, attempting as 
it were to escape ; but until the valvular opening of the vagina is opened or retracted 
by special muscles, the forward movements of the foetus only press the valve closer 
down, and shuts it more firmly against the side. 
At length, after one or two partial openings of the valve, the muscles withdraw it 
completely, and the ovisac contracting energetically at the same moment, the embryo 
is suddenly and forcibly expelled into the surrounding water. No sooner however 
does it escape than it begins to swim about with the peculiar semicircular movements 
