178 F. X. WILLIAMS 



seen to be almost in its original arcuate position. The yolk- 

 mass is becoming constantly smaller in proportion to the size 

 of the embryo. 



A Photuris embryo almost ready to hatch is shown in figure 

 45, lying in the final curled position. It now measures about 

 600 micra in the diameter of its curled position. Several days 

 before emergence it is capable of slow movement, which is plainly 

 visible, for the luminous organ, now functional, will be found to 

 change its position. 



The fate of the embryonic envelopes was not determined, no 

 dorsal tube (the remnants of the serosa) was found, though it 

 is probable that both the amnion and the serosa are eventually 

 absorbed into the dorsal part of the body. 



The mesenteron or midgut is much distended with the greater 

 part of the deeply-staining yolk material, so that the freshly- 

 hatched larva begins life with a full stomach. 



THE PLEUROPODS OR FIRST ABDOMINAL APPENDAGES IN THE 

 EMBRYOS OF PHOTURIS AND PHOTINUS 



In both Photuris and Photinus, the pleuropods, so named by 

 Wheeler C90) are very conspicuous by reason of their size and 

 structure. In neither genus was the organ observed in the first 

 stages of development. In a Photuris embryo about one-third 

 developed and before the enveloping amnion has been ruptured 

 (fig. 39), the appendage, I A, is a knob-liked process with a sort, 

 stout peduncle, and, like the thoracic legs, with its axis some- 

 what inclined caudally. It is a little less in diameter than the 

 length of the segment from which it originates. In a sagittal 

 section through the organ, its cells, which are of course hypo- 

 dermal, are in large measure, strongly differentiated in that they 

 are columnar, with larger, very elongate nuclei, three or more 

 times as long as wide, the whole forming a somewhat convex 

 layer or disc (fig. 54). At the distal extremity of the pleuropod 

 there is already much granular secretion, which is evidently extra 

 cellular and held in situ by the thin enveloping membrane, C. 



A somewhat later embryo (fig. 41) shows the organ IA, con- 

 siderably enlarged, its disc broader and flatter and the neck 



