Rathke, Growth of the Eggs of the Crawfish. 247 
anteriorly; and the labrum had also commenced its growth, in the shape 
of a flattish elevation with a darker margin, occupying the middle space 
between the foremost antenne. No trace of nervous or vascular system 
could be detected during the whole of this period. The embryo, it 
should here be observed, has by this time extended itself over about one 
fourth part of the surface of the vitellus, but its thickness is still incon- 
siderable. 
In the second period of developement, treated of in the fourth section, 
which closes with the appearance of the heart, and occupies a space of 
about a fortnight, from the end of April to the middle of May, the 
increase in the size and number of parts proceeds with much greater 
rapidity. The central piece enlarges itself to such an extent as fully to 
equal one eighth part of the surface of the vitellus, and at the same time 
acquires a considerable thickness; while the marginal piece, still remaining 
extremely thin and perfectly transparent, extends itself over the whole 
remaining part of that organ, and uniting its opposite edges, forms. with 
the central piece, a new and supplementary envelope. The pro- 
duction of the remaining external organs is continued in the same direc- 
tion, namely from before backwards; and the developement of those 
previously produced gradually proceeds. The rudimentary antenne 
increase in length, become detached from the surface nearly to their 
bases, and have their extremities partially bisected by a notch. The 
mandibles also lengthen, and enlarge, but more particularly in their basal 
portion, which continues to be applied and attached to the common sur- 
face, after the separation of the rest. The labrum gradually recedes from 
its position between the anterior antenne, and takes its station between 
the posterior ; and a cavity is formed behind it, communicatiug with the 
commencement of the esophagus, which now becomes partially visible 
on dissection. Of the new parts, the eyes are the first that make their 
appearance. Up to this period the anterior half of the central piece, 
which produces the organs hitherto named, and which must now be 
regarded as the head, forms by far the largest portion ; but the relation in 
this respect is henceforward reversed, and the posterior half enlarges itself 
with much greater rapidity. The umbilicated process of the latter 
becomes lengthened into an apparent tail, which includes, however, both 
tail and abdomen; and the depression in its surface is converted into the 
