THE EMBRYOLOGY OF INSECTS AND ARACHNIDS. 27 



The supra-oesophageal ganglion was studied by transverse and longitudinal 

 sections. Figures LXXI, LXXII, LXXIII, LXIV, represent transverse sections of 

 the brain of an advanced embryo cut backwards from the anterior part of the 

 cesophageal invagination. On each side of the brain will be seen folds (AM in 

 figure) which correspond to the amniotic folds of insects. In the last transverse 

 section (Fig. LXXIV,) the outer limbs of the folds have separated from the inner 

 limbs and have united. The thicker inner limbs of these folds have not united 

 but in other respects they correspond completely to the true amnion of the insect 

 embryo. 



It will be seen from a study of longitudinal sections (Figures LXXX, LXXX' 

 LXXXI, LXXXIP, LXXII) that these folds occur in the head region only, and cover 

 but a part of the supra-oesophageal ganglion. 



Figure LXXX, is a drawing of a longitudinal section of an advanced embryo 

 laterad of the median line. Figure LXXX' represents the cephalic portion of the 

 same section drawn with a higher power. AM in the figure represents the so-called 

 amniotic folds in cross section. No. I is the anterior portion of the supra-cesopha- 

 geal ganglion. No. I' is the posterior division of the same ganglion. MD repre- 

 sents the mandible. The portion of the brain above it represents the mandibular 

 division of the supra-oesophageal ganglion. 



The large cells (B, C, in the figure) with granular protoplasm and well defined 

 nuclei may be termed blood cells. On the fusion of these cells their protoplasm 

 appears to form blood plasma; and their nuclei, with an investment of protoplasm 

 in some cases, the blood corpuscles 



Figure LXXXI represents a section of the same series farther towards the 

 median line of the embryo. 



LXXXI' is a drawing of the same section more highly magnified. 



Here it will be seen that the inner amuiotic fold has separated from the outer 

 and adheres closely to the anterior portion of the supra-cesophageal ganglion. The 

 outer limb of the amnion fold extends over the supra-oesophageal ganglion to its 

 mandibular division. 



The blood cells are kept from passing out between the amnion folds by meso- 



