During the last stage of its continuance in th'e ovary the vesicle of Purkinje, disappears, and the first rudiments of the germ are disco vered. This series of changes might induce the belief that the germ is neither more nor less than the liquid of the vesicle shed upon the surface of the vitellus. Its form at first resembles that of a slight 3,vhitish cloud, which, by slow degrees, changes into an opaque white spot, well defined, and covering nearly the sixth part of the entire surface.
The egg is in the above state at the time it is received into the oviducts. These canals secrete an albuminous fluid, which surrounds the vitellus and its envelope, and which itself becomes covered with a membranous involu crum, called the chorion or dermoid envelope of the ovum. Another membrane still is thrown around the last, to serve as the means of attaching the ovum to the false abdominal extremities of the mother.
When the process of incubation begins the surface of the yolk is first seen to be come covered with star-like or serrated spots whitish in the first instance, and then white, which by-and-bye disappear entirely. The germ at the same time is extended uniformly over the whole surface of the yolk ; but again it seems to collect towards a point under the form of a white spot, which is the blastoderma. This spot, after undergoing certain variations in its form and dimensions, ends by becom ing elliptical with a slight furrow in its mid dle, shaped like a horse-shoe. This furrow soon extends ; its extremities meet, and its centre becomes depressed, so as to assume the appearance of.a. sacculus of some depth. The blastoderma enlarges at the same time, and presents the appearance of a cordiform spot. It is at the bottom of the sacculus but just mentioned, and in the nearest point of the blastoderma, that the first rudiments of organs make their appearance.
It is now that the orifice of the sacculus begins to enlarge ; the edges separate ; its bottom rises, so as at length to become pro minent, and a small nipple-like elevation ap pears upon it, hidden in some measure by the edge of the sac, which turns out to be the rudiments of the posterior portion of the body. At the same epoch there are formed anteriorly, on either side of the median line, two pairs of small strap-like bodies, which are by-and-bye discovered to have been the rudiments of the antenn, and another pair, which are the ear liest vestiges of mandibles. Between the two anterior antennee an azygous point presents itself, which is the rudiment of the labrum, and which, by the progressive development of the neighbouring parts, shifts by slow degrees to its final position between the second pair of antennm.
By slow degrees the blastoderma, the pe ripheral portion of which is much thinner and more transparent than the middle portion, is seen to extend on the surface of the vitellus, and at length to envelope it completely. Du ring this time the three pairs of spots which represent the antennw and the mandibles are growing larger, their edges becoming distinctly defined, and their extremities are receding from the surface of the blastoderma, under the form of a little cylinder, the end of which before long divides into two. After the an tennw have been seen, the peduncles of the eyes make their appearance, and detach themselves by degrees from the blastoderma, as. the pre ceding appendages had done. The nipple-like projection which we have seen formed at the bottom of the small blastoderrmc sac. enlarges at the same time, and assumes the form of an elongated lamina, the free end of which is turned forwards, and before long advances nearly to the labrum.
In the space included between the mandibles and the fold formed by the abdominal lamina of the embryo, of which we have just spoken, we now perceive the rudiments of two pairs of jaws and of the first pair of maxillary extremi ties, then of the second pair of these latter or gans, and soon afterwards of the third pair. These appendages appear in the same man ner as the antennw, and in proportion as they are evolved, the fold that marks the origin of the caudal lamina of the embryo recedes from the anterior part of the body; by little and little the basilar portion of the lamina becomes straightened, so as to gain the same plane as the remainder of the blastoderma, whilst its terminal portion continues bent underneath against the former. The five pairs of ambula tory extremities make their appearance succes sively in the same manner as the antennm and the oral appendages ; the same may be said with regard to the abdominal extremi ties ; and whilst this formation is going on, the annular divisions of the abdominal portion of the body are observed to be evolved. The carapace at length begins to be formed in the manner already indicated, and the ex tremities, as they sprout, alter their shapes, and become more and more unlike one another, as they approach the term of their embryotic development.