l8 THE EMBRYOLOGY OF INSECTS AND ARACHNIDS. 



of the letters will explain the details of the figures. C" in the figure, is the cross 

 commissure of the supra-cesophageal ganglion, crossing above the oesophagus. At 

 a point on the dorsal surface of this region of the embryo the yolk spherules lose 

 their definite outlines, and appear to run into a homogeneous mass, (YS in Fig.), in 

 which are cells, which are, apparently, migratory mesoderm cells. 



Figure XLVIII represents a section of the supra-cesophageal ganglion back of 

 the section just described. It shows the position of the antennae and labrum. 

 These correspond in every way to the antennae and labrum of other insects which 

 have been described. 



The amniotic folds of opposite sides have not met in this region. It will be 

 observed that the amnion at this stage does not cover the ventral surface of the 

 embryo, but has been apparently absorbed at all points except the dorsal extremity 

 of the body wall, where it persists as shown in the figures. 



Figure XLJX represents a section, posterior to those previously described, 

 through the anterior part of the oesophagus. I in the figure represents the inner 

 layer given off from the median line at this point. It has been carried inwards by 

 the oesophageal imagination. 



Figure L is a drawing of a succeeding section in which the circum-cesophageal 

 commissure of the supra-cesophageal ganglion is represented as surrounding the 

 oesophagus. In succeeding sections this commissure disappears. 



In figure LI it will be observed that the mesodermic ingrowth pushed in with 

 the inward growth of the oesophagus forms a hollow thickening on its dorsal surface. 

 This mesodermic thickening may be the rudiment of the heart in the head region, 

 as it apparently corresponds to a similar thickening which forms the head portion 

 of the heart in Thyridopteryx. 



Figures LII-LVII represent sections through the mandibular region. In figures 

 LV, LVI, LVII, it will be noticed that the amniotic folds have met and united on 

 the dorsal surface of the body. In figure LV the thickening of mesoderm on the 

 dorsal surface of the oesophagus previously referred to as the possible mass from 

 which in the head region the heart originates, is again shown. 



In figures LV, LVI, LVII & LVIII, the ganglion, marked No. II, is the man- 

 dibular portion of the sub-oesophageal ganglion. Figures LIX, LX, LXI, represent 



