From this time the egg be came every day larger until the twentu-fifIli day, when it was greatly distended and began to assume a kidney-shaped appearance, and commenced bursting longitudinally in the me dian line of the dorsal surface, the back of the soft and perfectly white embryo gradually pressing through the opening.
In the second period of developement the embryo is exposed to a new medium, and perhaps derives the means of its further growth from external sources, although it is still enve loped in the foetal membranes and retains its connection with the shell.
The liberation of the embryo is a remarkably slow process as compared with the escape of other animals from the egg. In Mr. New port's observations, from ten to twelve hours elapsed before the body of the young my riapod was so far liberated as to remain only partially enclosed between the two halves of the shell, as represented in fig. 318, being still at tached to its interior by a pe dicle or funis (fig. So remarkable is its condi tion at this period that it strongly resembles the ex pansion of the germ in the seed of a plant, rather than the evolution of a living animal. The embryo is per fectly motionless, and the bursting of its shell appears to be effected not by any direct effort of its own, since, up to this time, it has acquired only the form and external sem blance of a living animal ; but, by the force of ex pansion of the growing body, the development of which being greatest along the dorsal or larger curvature, exerts, in consequence, a greater de gree of force against the middle of the dorsal than the corresponding part of the ventral sur face; the head and tail of the embryo acting as a fulcrum against the ventral surface only at the ends of the shell, and thus bending it into the kidney-shaped form it assumes, while the dorsal surface of the embryo is gradually pressed through the opening. From the com parative rapidity of its enlargement imme diately after the shell is fissured, Mr. Newport observes, that it seems as if the stimulus given to it by exposure to a new medium, atmo spheric air, were the great means of exciting its evolution.
The embryo is now formed of eight distinct segments (fig. 319), including the head, the ninth or anal segment being still indistinct. The head is more defined in its out line, and firmer in texture than other parts of the body, and is inflected against the under surface of the pro thorax (2) or second egment, from which it is divided on the upper part by a deep transverse line : at its sides it exhibits a faint. trace of the future antenna'. The four tho racic segments also exhibit on their ventral sur face little nipple-shaped extensions, three of which on each side are the rudiments of future legs. When viewed from above, the body of the embryo appears compressed and wedge-shaped, its greatest diameter being in the second and third segments, while each succeeding segment is more and more contracted. Mr. Newport was unable at this period to detect any separate internal organs, the whole embryo being still a congeries of vesicles or cells, in the midst of which there seemed to be some faint traces of the commencement of an alimentary canal.
" At the end of the first day," continues Mr. Newport," I carefully removed the embryo and shell into diluted spirits of wine, and, on ex amination beneath the microscope, found the body of the embryo covered with an exceed ingly delicate cuticle, through which the cells it was formed of were distinctly visible. It was also completely enclosed in a smooth and perfectly transparent membrane (fig. 319, c), which seemed to contain a clear fluid, inter posed between it and the embryo. This mem brane 1 regard as the analogue of the amnion, the vitelline or investing membrane of the embryo in the higher animals, and identical with the membrane vitelli, before described, as the proper membrane of the yelk in the egg of Julus. It is a shut sac that completely invests the embryo, except at its funnel-shaped ter mination at the extremity of the body (jig. 319, d), where it is constricted, and together with another membrane (e), which in the unburst egg is external to this and lines the interior of the shell, assists to form the cord or proper fun is (d).