520 KNOWER. [Vol. XVI. 



19). When the amnion is about to close over, the cells of this 

 end have drawn together and become incorporated in the disc. 



I cannot determine whether the concentration, in the early- 

 stages, is accomplished by the migration of independent amoe- 

 boid cells toward the embryonic area, or whether the blastoderm 

 outside this area is from the first a continuous membrane of 

 loosely connected cells which contracts toward the center of 

 the germ-disc. I believe, however, the blastoderm cells beyond 

 its limits to be independent, to a late stage in the formation of 

 the disc. 



A less marked concentration of the surface cells has been 

 observed in other insects in similar stages, resulting in a closer 

 approximation of the cells of the embryonic area. Refer to 

 Patten (21), Figs, i and 2 of PI. XXXVI {A), and Fig. 5 of 

 PI. XXXVI {B), and Wheeler (25), Figs. 63, 64, 66, and 68. 



In the Termite's Qgg, where the embryo is a comparatively 

 small disc when completely established, the concentration to 

 establish this disc is an especially notable process. 



McMurrich (18) has discovered a similar method of the for- 

 mation of the embryonic rudiment in Isopods. His figures, 

 17-19 and 50-52, show the formation of the germ-band in 

 these Crustacea by a concentration of the surface cells toward 

 the ventral side of the tgg. He finds an intimate connection 

 between this phenomenon and the formation of an " under- 

 layer," and my observations on the Termite's o.g'g lead me to a 

 similar conclusion for it. Hence the detail in which I have 

 described the early stages. 



Origin of the Mesoderm. 



I have studied the origin of the under-layer with especial care, 

 on account of the recent conflicting results of Wheeler (26) and 

 Heymons (14) in regard to its formation in the Orthoptera. 



In the Termite there is no gastrula invagination. The 

 under-layer begins to appear at an early stage in the formation 

 of the disc, somewhat earlier than PI. XXX, Fig. 14, when its 

 cells first begin to be crowded. During this period, at irregu- 

 lar points in the embryonic area, lateral as well as median, 



