CRUSTACEA. 



785 



pair of membranous sacs which contain and 

 transport them from place to place. These 

 varieties in the accessory organs of gene- 

 ration, are in many cases sufficient to distin- 

 guish the sexes : thus, among the Decapoda 

 brachyura, the females are known at a glance 

 by their wider abdomen, which is sometimes 

 of such dimensions as to cover almost the 

 whole sternum. Sometimes these sexual diffe- 

 renceti extend to the antennae and to various 

 other organs; sometimes it even influences the 

 size, and occasionally, as we have said, the 

 general external conformation is modified to 

 such a degree, that the male and the female 

 of oire and the same species have been taken 

 as types of two distinct genera. There are 

 some species jsf which the females only are as 

 yet known to naturalists. 



The ovum appears to be formed in the walls 

 of the ovary, from whence it is detached when 

 it has attained a certain size, and falls into the 

 cavity of the organ. We have already stated 

 in what manner it is expelled, and in what 

 mode fecundation is accomplished in its pas- 

 sage through the oviducts, or after its extru- 

 sion. The distinguished German naturalist, 

 Rathke, has given particular attention to the 

 divers phases of the evolution of the egg of 

 the Astacus fluviatilis, as well before as after its 

 escape from the ovary and oviduct; and we 

 believe we cannot conclude this article more 

 satisfactorily than by presenting our readers 

 with a simple and brief analysis of his work.* 



The first and earliest form under which the 

 ovum meets the eye in the ovary is that of a 

 transparent vesicle, its walls of extreme te- 

 nuity, and filled with a watery fluid. This is 

 the vesicle of Purkinje. By-and-by there is 

 another membranous and very thin envelope 

 formed all round this vesicle, and in the minute 

 interval that separates the two coverings there 

 is a second fluid deposited, transparent like 

 the other at first, but soon becoming opaque, 

 whitish, and viscid; this is the vitellus or 

 yolk. As this increases in size, the vesicle of 

 Purkinje, which still preserves its first dimen- 

 sions, quits the centre, and goes to be attached 

 to the circumference, which, at last, it almost 

 touches at one point. During this time the 

 vitellus or yolk is continually declining in 

 transparency, on account of the formation 

 of an infinity of globules, which, at length, 

 transform it into a viscid mass of a deep brown 

 colour. 



During the last stage of its continuance in 

 tfte 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 whitish 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 



* Untersuchungen ueber die Bildung und Ent- 

 wickelung des Flusskrebses, fol. Leipz. 1829. 



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 

 antennae, and another pair, which are the ear- 

 liest vestiges of mandibles. Between the two 

 anterior antennae 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 

 antennae. 



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 antennae 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- 

 tennae 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 blastoderrnic sac enlarges 

 at the same time, and assumes the form of an 



