No. 3-] THE EMBRYOLOGY OF A TERMITE. 523 



is quite sharply marked out, especially on its posterior border 

 (PL XXX, Figs. 16 and 17). It is along this border that the 

 amnion is to appear. PI. XXX, Fig. 18, with the two figures 

 just referred to, shows that, as concentration of the embryonic 

 area proceeds, the nuclei at the posterior end draw together into 

 the disc in concentric rows, which results in a closely crowded 

 semicircle of cells that becomes quite conspicuous in surface 

 views. In PI. XXX, Fig. 18, this semicircle has become a 

 band of nuclei, much darker than the region of the disc just in 

 front of it, where the nuclei are not so densely crowded. 



Sagittal sections of discs in these stages (PI. XXXI, Fig. 32), 

 younger than that illustrated by PI. XXXI, Fig. ^'i^, teach that 

 the posterior margin, corresponding to the dark semicircle on 

 the surface, differs from the rest of the disc only in a some- 

 what greater thickness of the ectoderm. There is as yet no 

 fold in sections. 



It is evidently the posterior thickened margin of Figs. 18 and 

 32, which has folded over in Figs. 19 and 33, to become the 

 amnion. 



It will be noted then, in reference to the origin of the amnion, 

 that it is formed with the disc in the same process of concen- 

 tration, and that it is, at first, evidently merely a specialized por- 

 tion of the disc before folding forward to become the amnion. 



This agrees essentially with the figures which Bruce (6) 

 gave for Mantis (PI. IV, Figs. 42 and 43); with Patten's (21) 

 description and figures of the Phryganid ; with Will's (27) 

 account of the Aphids ; and with the results of most observers, 

 though all do not agree in regarding the amnion as a part of 

 the embryonic rudiment. 



/ have reserved a final section of this paper for a general dis- 

 cussion of the origin of tJie membranes iji insects. 



Continued Growth of the Amnio-Serosal and Mesodermal 

 Rudiments to the Closure of the Amniotic Cavity. 



Preparations of eggs illustrating successive stages in the 

 closure of the amniotic cavity show that this is accomplished 

 by the single semilunar fold growing forward from the posterior 



