86 EMBRYOLOGY. 



At about this period the yolk sac is more circumscribed 

 than before in the germ. Whether there is an internal 

 division of the embryonal membrane (visceral mem- 

 brane) which forms the yolk sac, as in Agrion and the 

 Hemipterai I was not able to determine. The outer 

 layer (' amnion,' or parietal layer) which surrounds the 

 embryo I did not at this stage observe, but as soon as 

 the appendages are formed, as in fig. 8, the chorion 

 bursts on slight pressure, and the embryonal mem- 

 brane is readily detected, enveloping the embryo, like 

 the ' larval skin ' of many Crustacea. 



'' At the period indicated by fig. 6 the dorsal walls 

 (tergites) of the arthromeres are closed in and the 

 rudiment of the spring (figs. 6 and 7, sp.) appears. 

 It is liable to be mistaken for the antenuse (I), so large 

 and well developed is it. It is evidently, in all respects, 

 both in its origin from the under side of the penulti- 

 mate segment of the abdomen and in its form, homo- 

 logous with the cephalic and thoracic appendages. In 

 fig. 7 the mandibles are just behind, and but little 

 smaller than the antennse (I), and in a vertical view 

 (fig. 9) of the embryo when somewhat older, a pair of 

 tubercles (II) are seen next to the rudimentary antennse 

 (I), which are probably the rudiments of the mandibles. 

 In fig. 10 i indicates the position of the rudiments of 

 the alimentary canal ; the yolk-cells composing it are 

 much smaller than those scattered over the other 

 portions of the body and in the appendages. Fig. 11 

 shows the antennae of much greater length than before, 

 with the rudiments of the articulations scarcely indi- 

 cated. At a later period the antennco especially seemed 

 to show traces of articulations, and have grown much 

 longer, while the end of the abdomen is divided deeply 

 by the median furrow into two lobes. The mandibles 

 (II) and first maxilloe (III) are distinct. I was unable 

 at this or any other period to discover any traces of the 

 second maxillae. Though existing in a very rudi- 

 mentary state in the adult, I could not detect them 

 after repeated attempts, but do not doubt but that 



