No. I.] CONTRIBUTIO.W TO lA'SECT EMBRYOLOGY. 37 



Bruce ('86) appears to have been the first to describe the 

 origin of the mesentoderm from a median ingrowth of the 

 germ-band in the Orthoptera. The species which he studied, 

 is, I have every reason to believe, Stagmomaiitis Carolina. 

 His description is very meagre and his figures are unsatis- 

 factory. 



More convincing are Graber's figures and description of 

 mesentoderm formation in Stenobothrus variabilis {'88, PI. 

 XIV, Fig. 11; PI. XV, Fig. 13). His Fig. 11 shows that 

 there is in the median hne a distinct infolding of the ventral 

 plate cells — a true invagination. In a more recent paper ('9o), 

 the account is briefly repeated without any important additions. 



In his recent study of the embryogeny of Blatta germanica, 

 Cholodkowsky ('9i^) gives an account of the formation of the 

 germ-layers more in harmony with what we know of the process 

 in the Coleoptera than the account which I gave. But he has 

 not come to any definite conclusion respecting the formation of 

 the entoderm, and although he maintains that there is a distinct 

 blastoporic groove running the length of the germ-band, he 

 does not figure it in surface view, and most of his sections 

 betray such an amount of distortion in his preparations that 

 one may hesitate to regard the slight depressions in his figures 

 (Figs. 7, 8, 10, etc.) as indicating invagination. Nevertheless 

 I believe from renewed study of the Orthoptera, that Cholod- 

 kowsky is correct in deriving the mesoderm from a median 

 proliferation of the primitively one-layered germ-band, and the 

 entoderm from two formative centres — one in the oral and 

 one in the anal region. 



In XipJiidiiim, soon after its first appearance, the blastoporic 

 depression, when seen from the surface (Fig. i), is a straight 



refractory from a technical point of view. The cells of the embryo are often 

 smaller and less distinct than they are in the Metabola. Moreover, the great 

 quantity of yolk and its singular brittleness in hardened specimens renders 

 paraffin sectioning most unsatisfactory, and rather than incur the great expend- 

 iture of time which working with celloidin involves, the student gladly selects 

 some Coleopteran or Dipteran egg which is all that can be demanded from a 

 purely technical point of view. Nevertheless the Orthoptera constitute, by com- 

 mon consent, one of the most primitive orders of the Insecta ; their eggs are 

 large and may be readily procured in great numbers ; their development is so 

 gradual that all the requisite stages may be obtained without the least difficulty. 



