36 WHEELER. [Vol. VIII. 



described in XipJiidinvi. The embryo of Orclielininm passes 

 through the yolk in the same manner as the XipJiidiiim 

 embryo, shortens on the dorsal yolk, then grows apace, moves 

 around the lower pole and finally begins the yolk-enveloping 

 process on the ventral surface of the egg in the same way 

 as the XipJiidinvi embryo. It also develops an indusium 

 which is set free from the head and spreads over the yolk 

 while the embryo is passing through it backwards. In OrcJicl- 

 imum the inner indusial layer also secretes a brownish pigment- 

 like substance which enables one to follow its movements as it 

 gradually covers more and more of the yolk. A clear slit is 

 likewise left on the dorsal surface between the folds of the 

 organ. But in the time of closure of this slit OrcJicliniuni 

 differs from XipJiidinvi. In the latter insect we found that 

 the slit closed soon after the embryo had straightened on the 

 dorsal yolk, before it had shortened very decidedly. In 

 OrcJiclimmn the closure is considerably delayed. The embryo 

 shortens, then grows in length and breadth, passing beyond 

 Stage G of XipJiidinvi and its head nearly reaches the lower 

 pole before the two folds of the indusium meet and fuse. 

 Frequently in this stage, when the embryo is about to revolve, 

 the polar ends of the slit are still open, the membranes having 

 fused over the embryo. In a little later stage, however, the 

 indusium has completely enveloped the yolk. 



II. Remarks on Gastrulation in the Orthoptera. 



Although many important observations have of late been 

 contributed to the embryology of the Insecta, our knowledge of 

 the formation of the germ-layers in the Orthoptera cannot be 

 said to have made any signal advance. As late as 1889 so few- 

 forms of this order had been studied that I felt justified in 

 expressing some doubt as to whether their mesentoderm was 

 formed in the same manner as in the higher Metabola (Cole- 

 optera, Diptera, Lepidoptera). My doubts were confirmed by a 

 study of BJatta, when I failed to find the oral formative centre 

 of the entoderm ('89^).^ 



1 We need not go far to seek the reasons for this gap in our comparative 

 studies. The eggs of the Orthoptera are almost without exception extremely 



