346 



INSECT A. 



gives rise to the whole embryo. 



E 



OF 



FIG. 190. A 

 PLATYGASTEK. 



SERIES OF STAGES IN THE DEVELOPMENT 



(From Lubbock ; after Gauiu. ) 



The two peripheral bodies increase by 

 division, and soon ap- 

 pear as nuclei imbed- 

 ded in a layer of pro- 

 toplasm (fig. 190 D, 

 E, F). The layer so 

 formed serves as a 

 covering for the em- 

 bryo, regarded by Ga- 

 nin as equivalent to 

 the amiiion ('? serous 

 membrane) of other 

 Insect embryos. In 

 the embryo cell new 

 cells are stated to be 

 formed by a process 

 of endogenous cell for- 

 mation (tig. 190 D, E). 

 It appears probable 

 that Ganin. has mis- 

 taken nuclei for cells in the earlier stages, and that a blastoderm is 

 formed as in other Insects, and that this becomes divided in a way 

 not explained into a superficial layer which gives rise to the serous 

 envelope, and a deeper layer which forms the embryo. However this 

 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 cephalothuracic 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 cephalothoi-ax there is first formed a pair of 

 claw-like appendages 011 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 antenna? 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 arunion, and pre- 

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

 spoken of as the Cyclops larva by Ganin. 



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

 sented in fig. 191 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, Ifg), the development of 



