Packard.] 138 [Nov. 19, 



about .01 inch in diameter. The chorion is thin and structureless, 

 but covered by an outer shell, which renders the egg quite opake, so 

 as to prevent one from making out the earlier embryonic changes. 

 It is evidently secreted and laid upon the chorion just before the egg 

 is laid. Its surface is pitted with elongated hexagonal depressions, 

 which cross the egg transversely. Compared with the eggs of Calli- 

 phora vomitoria, the Meat Fly, they are rather smaller, those of the 

 latter being .06 inch in length, and thicker in proportion, while the 

 hexagons are in the Meat Fly larger and longer. The eggs of both 

 flies are dull chalky white. 



The micropyle is round, and easily perceived on twisting the egg 

 under the compressor. 



The yolk granules are exceedingly fine, as in C. vomitoria, so that 

 the yolk is quite dense. 



Owing to the density of the exochorion, as we may term the outer 

 shell, it was impossible to observe the earliest embryonic changes, 

 the formation of the blastoderm, and the primitive band. From one 

 specimen hardened in alcohol, we succeeded in tearing off the exo- 

 chorion by aid of the compressor and needles, an operation which we 

 could not succeed in doing on recent eggs, as they invariably broke. 

 This stage apparently agrees with that of CallipJiora vomitoria, fig- 

 ured by Weismann on Taf. v., figs. 65, QQ. 



A more advanced stage, as seen through the exochorion, is repre- 

 sented by Weismann's figures 71 and 72. 



At a more advanced stage (PL 3, Fig. 1) the embryo is nearly fully 

 formed. The segments and locomotive spines crossing the under 

 side of each segment are formed. The head is indicated; the two 

 tubercles probably indicating the 1st maxillae; the head is much 

 larger compared to the size of the prothorax, than subsequently, and 

 the dark Y-shaped twin spots indicate the formation of the oesopha- 

 geal frame work. The anal spiracles are formed, but the spiracles are 

 still somewhat rudimentary, though the tracheae leading from them 

 appear to be fully formed. 



Just before the embryo hatches, the body becomes more transpar- 

 ent, and, as seen in Fig. 2, the main tracheae can be traced through 

 their whole length, with their lateral and cephalic branches. The 

 rows of locomotive spines are very distinct. They can be easily seen 

 in the egg in its natural state, but the figure represents the living egg 

 with the exochorion removed, this covering having already split 

 along the under side, just as we have seen it in C. vomitoria. 



