No. 3-] THE EMBRYOLOGY OF A TERMITE. 52 1 



some of the cells are pushed below the surface by the concen- 

 tration of the blastoderm. Other cells are separated toward 

 the under surface of the ectoderm, by tangential divisions of 

 its nuclei, at various scattered points (PI. XXXI, Fig. 30). 



As these processes continue, the under-layer constantly 

 gains in bulk. Its formation is to be traced back to the con- 

 centration of the cells of the disc, and when this has reached the 

 stage represented by PI. XXX, Fig. 18, the under-layer cells 

 have for the most part collected into a plug projecting into the 

 yolk. From the surface this plug appears as a darkened area 

 of crowded nuclei near the center of the disc. 



Preparations of a series of discs, after the under-layer has 

 become thus crowded into a plug, illustrate the growth of this 

 collection of cells. PI. XXX, Figs. 16-19^ show, in surface 

 views, the gradual extension of the plug, up to the time when 

 the amnio-serosal fold has grown well forward over the disc. 



Sections through these stages and those just preceding and 

 immediately following, taken in connection with what has been 

 learned from surface views, give interesting data as to the for- 

 mation of the under-layer and the amnion, 



PL XXXI, Fig. 30, gives a cross-section of the single- 

 layered disc at a stage somewhat older than PL XXX, Fig. 13, 

 when it is first definitely outlined from the surrounding blasto- 

 derm. There is a crowded appearance of the cells, and some 

 of the nuclei are displaced from the surface and seen wedged 

 below. At various points in the surface layer, at the sides as 

 well as near the middle, nuclei are also found in the act of 

 dividing toward the lower surface, thus adding to the number 

 of cells adhering in the lower layer of the disc. 



A cross-section (PL XXXI, Fig. 31) of the embryonic area 

 through the region of the plug at the stage (PL XXX, Fig. 18), 

 when compared with PL XXXI, Fig. 30, cutting the same 

 region of a younger disc, shows that the plug has grown con- 

 siderably by the gradual addition of cells from the ectoderm 

 and their subsequent multiplication. The mesodermal plug is 

 still in close continuity with the ectoderm. 



A sagittal section of a disc of this age (PL XXXI, Fig. 32) 

 shows the plug quite distinctly. 



