No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. 39 



applied to the inner surface of the germ-band. The process 

 whereby the inner layers are formed is, therefore, a slurred 

 invagination. In this respect XipJndhim resembles Blatta. 



The further differentiation of the mesentoderm is quite as 

 difficult to follow in XipJiiduuii as in other Orthoptera. In 

 these stages the embryo cannot be satisfactorily isolated from 

 the yolk and sectioned by itself, and so friable is the yolk that it 

 is almost impossible to obtain thin sections through the entire 

 ^gS by the ordinary methods. After studying a few series of 

 sections obtained by means of the celloidin method I can, how- 

 ever, affirm that the invaginated cells give rise to both 

 entoderm and mesoderm. The former has a bipolar origin, as 

 has been made out in the higher forms ; in Apis by Grassi ('84) ; 

 in Hydrophilus by H eider ('89) ; in Doryphora by myself ('89) ; 

 in Musca by Voeltzkow ('89) and by Graber ('89); and in 

 Chalicodoma by Carriere ('90). The anal is considerably larger 

 than the oral formative centre and its elements seem to arise 

 in part from the bifurcation and in part from the deeper 

 portion of the blastopore just in front of the bifurcation. 



In Xiphiduim, just as in the higher Metabola, a pair of 

 entoderm-bands grows towards the baso-abdominal region from 

 either entoderm-pole. Each band, consisting of only one 

 layer of much-flattened cells, meets that of its respective side 

 and then begins to envelop the yolk by proliferation at its 

 ventral and dorsal edges. Transverse sections show that 

 at first the bands are only two or three cells in breadth and that 

 these are closely applied to the dorsal faces of the mesomeres 

 which are formed by this time. 



I have made no observations on the relations of the procto- 

 dceum to the posterior end of the blastopore, but in regard to 

 the anterior end and its relation to the stomodseum my results 

 are more definite. Figs. 32-34 represent three successive 

 sections through the head of an embryo in Stage D. The last 

 section (Fig. 34) passes through the stomodaeum (st.) which is 

 just forming as a rounded depression in the cephalic ectoderm. 

 Its large columnar cells are regularly arranged and have their 

 nuclei in the inner ends. The next section (Fig. 33) passes 

 just in front of the stomodasum and cuts two masses of cells in 



