No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. 



2. The Formation of the Embryo and its Backward Passage 



Through the Yolk. 



a. SURFACE CHANGES. 



The sub-opaque, cream-colored ^^g of Xiphidiiim is elongate 

 oval, 3-5 mm. long and i mm. broad through its middle. One 

 of its poles is distinctly more attenuate than the other, and 

 there is a faint curvature in the polar axis which causes one 

 side of the Q.^g to be distinctly convex and the other distinctly 

 concave. The broader pole is the posterior, and is the first 

 to leave the vagina during oviposition; the attenuate pole is, 

 therefore, the anterior. In the galls the eggs stand with their 

 attenuate poles pointing upwards. The convex face of the Q.^g 

 is the ventral, the concave face the dorsal region. Inasmuch 

 as the Q.g^ undergoes no change in shape during development, 

 it is easy to orient the embryo in its different stages. This is 

 of considerable importance, as will appear from the sequel. 



The yolk is pale yellow and very similar in constitution to 

 the yolk of other Orthopteran eggs. It is enclosed by a thin 

 leathery chorion which suddenly becomes transparent on 

 immersion in alcohol. When dry it is white, and the creamy 

 color of the ^^% is due to the yellow yolk shining through. As 

 in Blatta, the chorion is the only envelope of the freshly laid 

 ^^■g ; what I described in a former paper ('90b) as the vitelline 

 membrane is in reality comparable to a " Blastodermhaut " as I 

 shall point out. 



The chorion varies somewhat in thickness at different points 

 in the ^gg, being ii/^ towards the middle and 19/x at the 

 poles. It is quite elastic and when cut curls in at the edges. 

 Its inner surface is very smooth, while outwardly it is covered 

 with round or oval projections which measure about 3. //a in 

 diameter. They are flattened at their summits and are placed 

 so closely together that only narrow channels run between 

 them and give the chorion the appearance of being covered 

 with a fine net of nearly uniform meshes. On closer examina- 

 tion it is seen that the projections are arranged in hexagonal 

 groups. These are very distinct at either pole but fade away 



