522 KNOWER. [Vol. XVI. 



Both surface views and sections of these stages agree in 

 exhibiting no gastrular groove. On the contrary, it is as I 

 have stated — the under-layer arises at all points in the germinal 

 disc, as a result of the concentration of this area and of the 

 tangential divisions of its cells. The formation of a mesoder- 

 mic plug is apparently a further outcome of the concentration. 

 (Consult McMurrich (i8) on the formation of the under-layer 

 in Isopods.) 



A discussion of the general bearing of these facts on the origin 

 of the mesoderm in insects will be foimd further on. 



Origin of the Amnio-Serosal Fold. 



I have devoted much attention to the early history of the 

 embryonic membranes, on account of the general interest their 

 presence excites. 



When the amnio-serosal fold is first clearly defined as a fold 

 in sections, it appears from the surface (PI. XXX, Figs. 19 

 and 192^) as a semilunar fold along the posterior border of the 

 embryonic disc, extending forward on either side toward the 

 anterior end. Sagittal sections of this stage make plain that 

 the inner or amniotic layer of the fold is not distinguishable 

 from the ectoderm of the germ-disc, except by its position 

 (PI. XXXI, Fig. 33). It is of the same thickness as the ecto- 

 derm, and its nuclei are arranged in the same layers, inverted. 

 The outer or serosal portion of the fold, on the other hand, is 

 quite different (PI. XXXI, Fig. 33). This is a thin membrane 

 of much flattened cells with nuclei far apart. This membrane 

 resembles the rest of the extra-embryonic blastoderm of which 

 it is a continuation. {This evident distinction between amnion 

 and serosa is important, as zvill appear further on.) 



Figs. 19 and 33 of Pis. XXX and XXXI, though represent- 

 ing the amnion when first appearing as a completed fold, do 

 not exhibit the earliest stage in the formation of the amnio- 

 serosal fold of the Termite. 



Several stages before a fold can be made out in sections, its 

 position is outlined on the surface of the disc. When the under- 

 layer plug first appears in surface views, the embryonic disc 



