41 8 INSECTA. 



may be, a differentiation into an epiblastic layer of columnar cells and 

 a hypoblastic layer of more rounded cells soon becomes apparent in the body 

 of the embryo. Subsequently to this the embryo grows rapidly, till by a 

 deep transverse constriction on the ventral surface it becomes divided into an 

 anterior cephalothoracic portion and a posterior caudal portion (fig. 190 F). 

 The cephalothorax grows in breadth, and near its anterior end an in- 

 vagination appears, which gives rise to the mouth and oesophagus. On 

 the ventral side of the cephalothorax there is first formed a pair of 

 claw-like appendages on each side of the mouth, then a posterior pair of 

 appendages near the junction of the cephalothorax and abdomen, and 

 lastly a pair of short conical antennae in front. 



At the same time the hind end of the abdomen becomes bifid, and gives 

 rise to a fork-like caudal appendage ; and at a slightly later period four 

 grooves make their appearance in the caudal region, and divide this part of 

 the embryo into successive segments. While these changes have been 

 taking place in the general form of the embryo, the epiblast has given rise 

 to a cuticle, and the hypoblastic cells have become differentiated into a 

 central hypoblastic axis the mesenteron and a surrounding layer of 

 mesoblast, some of the cells of which form longitudinal muscles. 



With this stage closes what may be regarded as the embryonic develop- 

 ment of Platygaster. The embryo becomes free from the amnion, and pre- 

 sents itself as a larva, which from its very remarkable characters has been 

 spoken of as the Cyclops larva by Ganin. 



The larvae of three species have been described by Ganin, which are repre- 

 sented in fig. 1 9 1 A, B, C. These larvae are strangely dissimilar to the ordinary 

 Hexapod type, whether larval or adult. They are formed of a cephalothoracic 

 shield with the three pairs of appendages (a, kf, lfg\ the development of 

 which has already been described, and of an abdomen formed of five seg- 

 ments, the last of which bears the somewhat varying caudal appendages. 

 The nervous system is as yet undeveloped. 



The larvae move about in the tissues of their hosts by means of their 

 claws. 



The first larval condition is succeeded by a second with very different 

 characters, and the passage from the first to the second is accompanied by 

 an ecdysis. 



The ecdysis commences at the caudal extremity, and the whole of the 

 last segment is completely thrown off. As the ecdysis extends forwards 

 the tail loses its segmentation and becomes strongly compressed, the 

 appendages of the cephalothorax are thrown off, and the whole embryo 

 assumes an oval form without any sharp distinction into different regions 

 and without the slightest indication of segmentation (fig. 191 D). Of the 

 internal changes which take place during the shedding of the cuticle, the 

 first is the formation of a proctodaeum (gfi) by an invagination, which ends 

 blindly in contact with the mesenteron. Shortly after this a thickening of 

 the epiblast (bsnt) appears along the ventral surface, which gives rise mainly 

 to the ventral nerve cord ; this thickening is continuous behind with the 



