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 antennae. 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 antennae 

 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 antennae, and takes its station between 

 the posterior ; and a cavity is formed behind it, communicatiug with the 

 commencement of the oesophagus, 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 



