NO. 3 EMBRYOLOGY OF FLEAS KESSEL 53 



the differentiation of the lower layer. Carriere (1890) believes these 

 cavities of the mesodermal somites originate in the same manner in 

 the development of Chalicodoma. According to the supposition of 

 these authors, the lumen of the original tube is incompletely and 

 temporarily closed by a dorsoventral compression. This closure flat- 

 tens the mesoderm into two distinct layers which are said to separate 

 at a later time in the regions of their lateral margins to form the 

 coelomic sacs. This explanation is in harmony with their designation 

 of the mesodermal tube as an elongated gastrocoel. Apparently, it 

 was an attempt to parallel the derivation of the coelomic pouches from 

 the archenteron, such as occurs in certain other animals, and to har- 

 monize the situation with the view of Hertwig and Hertwig (1881) 

 which prompted their hypothesis. Graber (1890) was not able to 

 substantiate Heider's statements as to the case of Hydrophilus, and 

 the work of Carriere has not been confirmed. In the case of flea 

 embryos there is no possibility of such an origin for the coelomic 

 sacs even in the posterior region of the embryonic rudiment where 

 a distinct mesodermal tube is formed, for the lower layer, when it 

 reaches the lateral margins of the germ band, is only one cell thick. 

 It is not until later that the paired mesodermal bands are produced, 

 and their appearance is due to cellular proliferation of the originally 

 single-layered mesoderm. In the Siphonaptera, therefore, the coelomic 

 sacs arise as independent clefts in the thickened and solid lateral 

 regions of the lower layer. 



The number of the pairs of coelomic sacs occurring in the embryos 

 of fleas appears to be 16. The most anterior of these lies in the deuto- 

 cerebral segment. No indication of their presence was observed in 

 either the preantennal or the intercalary segment. It appears that 

 such cavities have been detected in the preantennal segment of only 

 one insect form, viz, Carausius morosus (Wiesmann, 1926), and even 

 in this generalized species they are rudimentary. As for paired meso- 

 dermal cavities occurring in the intercalary segment, they have been 

 described in a few of the lower insects among which is Carausius 

 morosus according to the work of Wiesmann cited above. 



The 15 remaining pairs of coelomic sacs which occur in flea em- 

 bryos are located in the first 15 segments posterior to the stomodaeal 

 invagination. Three belong to the gnathocephalon, three to the thoracic 

 region, and the remaining nine pairs are found in the first nine 

 abdominal somites. The lumina of the most posterior pair are very 

 small and rudimentary. The last two segments of the abdomen 

 appear never to produce even such rudimentary cavities. 



