THE EMBRYOLOGY OF INSECTS AND ARACHNIDS. 19 



sections through the first pair of niaxillag. In this region the oesophagus terminates. 



The amniotic folds and ectoderm have not covered the dorsal surface of the 

 body, which is, however, closed by mesoderm cells. 



The supra-oesophageal ganglion extends through the first maxillary region of 

 the head. It will be noticed that the ganglion is separated from the superficial 

 ectoderm which is particularly thick on its dorsal surface. Possibly the compound 

 eyes are formed from this thickened portion of ectoderm. 



The maxillae have each two lobes outside of and at the base of the main axis 

 of the appendage. (XL- in Fig.) 



These lobes recall, though they are probably not homologous with, the exopo- 

 dite and epipodite of the Crustacean appendage. Similar lobes have been described 

 by Patten for the maxillary appendages of Blatta. 



Figure LXII is through the region of the second maxilla. The body wall is 

 here closed dorsally by mesoderm and the amniotic folds extend farther towards the 

 ventral surface than in preceding sections.- 



Figure LXIII represents a section through the maxillary region of what was 

 probably a somewhat older embryo. 



Figure LXIII is a portion of the same section more highly magnified. No. Ill 

 in the figure represents the ganglion of the second maxilla. I represents the meso- 

 derm which here as elsewhere covers the dorsal surface before the latter is closed by 

 the amnion and ectoderm. 



Beneath the dorsal mesoderm lie large cells with granular protoplasm and large 

 deeply granular nuclei. These may be described as blood cells (BC. in Fig.) In 

 the spider somewhat similar cells form the plasma and corpuscles of the blood. 



Figures LXIV and LXV represent sections through the thoracic region. These 

 figures hardly require explanation. It will be observed that the amnion, though 

 more extended than in preceeding figures of this series, does not cover the ventral 

 surface of the embryo. 



To sum up briefly the observations made on the embryology of the Orthoptera, 



v 



At one stage all the cells are probably on the surface at the bases of the yolk 

 pyramids. Yolk cells must then arise by migration from the surface cells. The 

 yolk cells probably take no part in the formation of the endoderm ; for when the 



