246 Analytical Notices of Books. 
_ The embryonal sacculus, from the time of its formation, gradually, but 
slowly, increases in size by the assimilation of the plastic matter of the 
vitellus, and changes its form to that of a more or less irregular ellipsis. 
A depression appears in its centre, usually in the direction of its longest 
diameter, which at first assumes the shape of a small segment of a circle, 
or of a horseshoe, but in the course of a few days increases in length, 
and approximates its two extremities. which at length unite together. 
This depression passes more and more deeply into the substance of the 
embryo, and a corresponding elevation of the latter extends into the 
vitellus, from which it receives a continued supply of plastic matter. 
During its formation the enlargement of the embryo at its edges steadily 
proceeds, the latter remaining, however, thin and transparent, while 
those parts which surround the depression are thicker and opake. After 
a time, the new growth at the edges puts on in two different places a 
clouded appearance, which rapidly increases, extending itself towards the 
entrance of the depression, and assuming an elliptical form. The two 
ellipses gradually approach each other, and at length form by their union 
a broad heart-shaped patch, the narrow end of which is in immediate 
apposition with the thickened portion surrounding the depression. The 
developement of the separate organs now commences. The antenne, 
labrum, mandibles and abdomen, first make their appearance, and nearly 
at the same moment. The last named part takes its origin from the 
depression ; but all the rest are produced from the surrounding parts of 
the embryonal sacculus, or, to speak more accurately, from the clouded 
heart-shaped patch. To avoid circumlocution the authour names the 
opake portion, the central piece of the embryo, distinguishing the depres- 
sion as its posterior, and the clouded patch as its anterior, half; while he 
denominates the transparent circumference, the marginal piece. By de- 
grees the entrance of the depression is enlarged, and its cavity is exposed, 
and at the same time brought more nearly to the level of the other parts. 
There is now seen upon its surface a small umbilicated elevation, the rudi- 
ment of the future abdomen and tail. At the same time there appears 
on the anterior half of the central piece of the embryo, on each side of 
the middle line, a process directed backwards and outwards constituting 
the commencement of the mandible. Two other pairs of similar pro- 
cesses, the rudiments of antenne, had previously become visible still more. 
