XII] 



EMBRYOLOGY 



235 



St. 



space between the two layers is the true coelome or primary 

 body-cavity. It is never very large in the Insecta. Its fate in 

 the Odonata is uncertain, but it is believed to give rise only to 

 the lumina of the gonads and the reduced pericardial space. 



It must be borne in mind that the segmentation of the embryo 

 (see below) extends to the mesoderm, subdividing it into blocks 

 forming the mesodermal somites, which correspond with the 

 external segmentation. The result of this is that the coelome is 

 constricted off into separate spaces or coelomic sacs. These have 

 not been studied in the Odonata. It also follows that the heart 

 and muscles, which are formed from the mesoderm, are formed 

 segmentally. 



The Endoderm. The origin of the lowest of the three germinal 

 layers, or endoderm, in the Odonata, is 

 still a matter of dispute amongst em- 

 bryologists. There can be little doubt 

 that TschuprofE is correct in stating that, 

 in the Odonata alone amongst insects, 

 the mid-gut is not formed entirely of en- 

 doderm, but consists at first largely of 

 vitellophags. This is clear from fig. Ill, 

 where the huge vitellophags (vit) are seen 

 to form the greater part of the middle 

 portion of the wall, while small nests of 

 endoderm cells (en) lie here and there 

 outside them. After hatching, when the 

 contents of the vitellophags are absorbed, 

 these cells themselves disappear, and the 

 endoderm spreads out to form the deep 

 columnar cells of the mid-gut. 



The difficulty, however, lies in deter- 

 mining the origin of these endoderm cells 

 themselves. We can only give our readers 

 the facts so far observed, and leave them 

 to choose between the rival theories. 

 Before, however, we can follow these 

 facts, we must trace the part played by 

 the ectoderm in forming the alimentary canal. 



pr. 



Fig. 111. L.S. through mid- 

 gut of embryo of Epi- 

 theca bimaculata Charp., 

 late stage, en nests of 

 endoderm cells ; m 1 -m 3 

 the three portions of the 

 mid-gut ; muse muscle- 

 layer; pr proctodaeum; 

 st stomodaeum; vit vi- 

 tellophag. After Tschu- 

 proff. 



