No. 3-] THE EMBRYOLOGY OF A TERMITE. 537 



all the families of the order, save the Phasmidae, an invaginate 

 gastrula has been found, and there can be little doubt that the 

 investigator who is so fortunate as to study embryos of this 

 family will find in them essentially the same process of germ- 

 layer formation. The view is now pretty generally held that 

 in the Insecta both mesoderm and endoderm arise from a 

 median longitudinal furrow (the former layer throughout 

 nearly the entire length, the latter only in the oral and anal 

 regions of the germ-band), and that vitellophags, or cells left 

 in the yolk at a time when the remaining cleavage products are 

 traveling to the surface to form the blastoderm, take no part 

 whatsoever in the formation of the mesenteron, but degenerate 

 m situ and finally undergo dissolution." 



I have been unable to obtain a copy of Heymons's study of 

 the germ-layer formation of Orthoptera and Dermaptera (14), 

 but his conclusions have appeared in abstracts and are as fol- 

 lows : The yolk-cells take no part in the formation of the 

 embryo. There is no true gastrulation process, but the under- 

 layer arises from all parts of the embryonic area. When what 

 is usually regarded as a typical gastrula invagination occurs, as 

 in most insects, it is to be explained, not as gastrulation, but 

 as a simple mechanical process caused by an aggregation of 

 cells at one point. The layer generally known as the mesento- 

 derm is in reality only mesoderm, the endoderm appearing rela- 

 tively late and arising from the ectoderm of the stomodeal and 

 proctodeal invaginations. 



My results agree with Heymons's conclusions as to tJie origin 

 of the mesoderm of insects primitively in a collection of cells 

 arising diffusely from the ectoderm ; but I must differ from him 

 and agree with Wheeler in the latter s interpretation of the 

 invaginate groove, from which the endoderm. and mesoderm, arise 

 in most insects, as a tnie gastrula. 



The Termite, which is certainly as primitive as any other 

 insect hitherto described, exhibits no gastrula invagination. I 

 have shown that the under-layer begins to appear at all points 

 in the embryonic rudiment at an early stage of its formation. 

 The plug of lower-layer cells, which becomes so prominent 

 as the germ-disc grows more distinct, is apparently largely 



