30 WHEELER. [Vol. V 1 1 1. 



length of the embryo and terminating in the pointed anterior 

 pole. A comparison of Figs. 8 and 9 shows that, although 

 the former embryo has completed its revolution, it is neverthe- 

 less in an earlier stage so far as the development of its organs 

 is concerned. This is particularly noticeable in the labrum, 

 antennae and mouth parts, the eyes and the saltatory legs. 

 Hence we may infer that the time for turning the lower pole 

 is subject to considerable variation. 



In Fig. 9 it will be observed that many of the abdominal 

 appendages have disappeared. Pairs are, however, retained on 

 the 8th to nth segments {ap^-cc, {ap'^'^)). The pleuropodia 

 are also still present though concealed behind the bases of the 

 metathoracic legs. The disappearance of the appendages on 

 the 2d-8th segments probably has its immediate mechanical 

 cause in the lateral stretching which characterizes these seg- 

 ments in their attempts to embrace the yolk. 



The embryo continues its growth as before in two directions 

 — the body constantly lengthening and thus bringing the head 

 nearer the pointed anterior pole, while its lateral walls, envelop- 

 ing more and more of the yolk, gradually grow towards each 

 other and finally unite in the median dorsal line. The union 

 begins with the 7th abdominal segment, just in front of the seg- 

 ments which have for some time been provided with a dorsal 

 wall, and continues headward. I am not certain as to what 

 becomes of the amnion during this process. Its cells appear 

 to take no part in the formation of the dorsal wall, but very 

 probably degenerate and become supplanted by the cells of the 

 advancing ectoderm. It must be remembered that a hard and 

 fast line cannot be drawn between the amnion and the pleural 

 ectoderm ; the cells of both structures passing into one another 

 by insensible gradations. My reasons for supposing that the 

 amnion proper takes no part in building up the embryo are 

 mainly of a theoretical nature and will be given in the latter 

 part of this paper. 



Concerning the fate of the inner indusium there can be little 

 doubt. While the embryo is continually advancing towards 

 the cephalic pole and enclosing more and more of the yolk — 

 this envelope, which, as above stated, is characterized by 



