THE EMBRYOLOGY OF INSECTS AND ARACHNIDS. II 



x 



which come together on the dorsal surface, over the alimentary tract, flatten out, and 

 adhere at their edges (Fig. XXXI). A more detailed description of figures XXVII, 

 XXXIII will make preceeding references clearer. 



Figures XXVII & XXVIII represent sections through the head. The head 

 ganglia are not observed in these figures, because the sections were made through 

 the ventral surface of the head outside the nervous substance. 



The oesophagus (OE in Figs.) is represented in cross section, as well as the 

 mandibles (MD) and labrum (LB). 



The cesophageal epithelium is represented as surrounded by mesoderm carried 

 in with it as it grew inwards from the median line. The mesoderm surrounding 

 the cesophageal ingrowth, on its dorsal surface, will be seen to be hollow, thus form- 

 ing what appears to be the first trace of the heart in the head region (H in Figs). 



A line joining numbers I and II in figure XVII, will represent approximately 

 the plane of the sections represented by figures XXVII and XXVIII. Succeeding 

 sections of the same series represented by figures XXIX, XXX and XXXI, were 

 cut in planes parallel and internal to the imaginary line joining numbers I and II 

 in figure XVII. In figure XXIX portions of the supra, and sub-cesophageal ganglia 

 are represented (Nos. I and II in Fig.) The sub-cesophageal commissure of the 

 supra-cesophageal ganglion is also represented (C in Fig. XXIX.) 



The structure marked GL in the figures is a prolongation of the salivary gland 

 which originates at the base of the mandibles (GL, Fig. XXVIII). Figure XXX 

 represents a section through a plane internal to that represented by the preceeding 

 figure. The section figured has been carried through the base of the brain, cutting 

 the circum-cesophageal commissure (C 1 in Fig.) 



Figure XXXI represents a section internal or dorsal to the preceeding, showing 

 the supra-cesophageal commissure of the supra-cesophageal ganglion (C" in Fig.) 



Figure XXXII is a drawing of a cross section of the embryo back of the 

 cesophageal ingrowth. The epithelium of the midgut (IE in Fig.) is represented 

 enclosing yolk cells and yolk spherules (YS, YC in Fig.) 



It may be here in place to describe the sense organs and appendages of the 

 head. The head which includes the portion of body containing the supra and sub- 

 cesophageal ganglia is distinctly separated from the thorax. 



