56 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 98 



the formation of two indistinct horizontal septa. The dorsal dia- 

 phragm is produced by the somatic mesoderm which, as stated above, 

 remains attached to the cardioblastic strands. The ventral diaphragm, 

 which is even less distinct than its dorsal counterpart, appears to be 

 developed from the ventral somatic mesoderm. 



Like the other structures of the circulatory system, the haematocytes 

 of flea embryos appear to be mesodermal in origin. They are all 

 nucleated cells. Wigglesworth (1934) is of the opinion that haemato- 

 cytes may play a part in the organogenesis of insects by producing 

 connective tissue membranes to cover the internal organs. In the case 

 of flea development the enclosing membranes, of the gonads at least, 

 are not formed in this manner. 



The embryonic blood cells in fleas seem to take their origin by pro- 

 liferation from the intermediate mesoderm between the longitudinal 

 mesodermal bands. Snodgrass (1935) suggests, however, that because 

 of their later phagocytic and digestive activities, it is possible that the 

 haematocytes of insects are really derived from the intermediate strand 

 of entoderm which is differentiated in some insects such as the 

 Isoptera (Strindberg, 1913). If this supposition is correct, they are 

 to be regarded as genetically related to the secondary trophonuclei and 

 the epithelial cells of the mesenteron. 



GONADS 



The origin of the germ cells and their migration to the epineural 

 sinus has been described in a previous section. Having arrived in the 

 region of the fifth abdominal segment, the germ cells, now gonia, 

 become apposed to the inner surface of the splanchnic mesoderm to 

 form two groups of cells, one on each side of the body. With the 

 rupture of the coelomic sacs and the inward movement of the splanch- 

 nic mesoderm toward the mesenteron the germ cells are carried along 

 farther toward the interior. Then again, with the dorsal migration of 

 the cardioblasts and until the separation of the splanchnic mesoderm 

 from the cardioblastic bands, the clumps of germ cells are moved 

 somewhat dorsally. During these changes in position the sex cells on 

 each side become enclosed by a covering of the splanchnic mesodermal 

 cells with which they are associated (pi. 12, fig. 89). These covering 

 cells gradually become flattened to form the follicular epithelium of 

 the gonad. Posteriorly, splanchnic mesodermal cells, similar to those 

 which form the epithelial envelope, differentiate into a strand which 

 is continuous with the gonad (pi. 12, fig. 90). This tube is the anlage 

 of the oviduct or vas deferens depending upon the sex of the larva. 



