14 THE EMBRYOLOGY OF INSECTS AND ARACHNIDS. 



seen on referring to the figures that on the union of the outer portions of the oppo- 

 site amniotic folds the sack enclosing the embryo will contain no yolk. 



How it happens that folds are formed by the dorsal growth of the amnion will 

 be understood by referring to figure XV. In describing the origin of the endoderm 

 reference was made to figures XV, XXIV, XXV, as illustrating stages in the forma- 

 tion of the rnidgut. 



In figure XV the most dorsal portion of the inner layer (IE in Fig.) on oppo- 

 site sides begins to be constricted off from the rest of the inner layer. 



The process of constriction has been completed in figure XXIV. The portions 

 of the inner layer thus constricted off (IE Figs. XXV, XXVI), on each side grow 

 together forming first the ventral surface of the digestive tract, thence they ex- 

 tend dorsally, and shut in the digestive tract on all sides. 



To sum up the embryology of Thyridopteryx : It was found that on the form- 

 ation of the blastoderm some of the cells, probably, do not reach the surface, but re- 

 main in the yolk as yolk cells, which take little, if any, part in the subsequent form- 

 ation of endoderm. 



The nervous system arises in the customary manner. The median ingrowth 

 between the nerve ganglia takes no part, as Hatscheck thought, in the formation of 

 the commissures. The supra-cesophageal ganglion is double. It has a double 

 commissure uniting its opposite halves. 



The ocelli probably arise from ectoderm independently of the nervous system. 

 The true amnion on the union of its folds forms a portion of the dorsal surface of 

 the body. Before the union of its opposite folds what might be described as a dorsal 

 organ is formed. 



NEUROPTERA. 



Chrysopa was the representative of this group which I studied. The obser- 

 vations made on the embryology of this insect, owing to lack of material, were very 

 incomplete. 



Figure XXXIII represents a transverse section of the head showing the position of 

 the upper lip and the antennae. The upper lip is distinctly bilobed. Its two lobes 

 are not, however, well shown in the figure. 



The abdomen of the neuroptera, according to Packard, consists of eleven somites 



