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 imite 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 antennae, 

 labrura, 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 umbiUcated 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 antennae, had previously become visible still more 



