52 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 98 



lescence of the last three ganglionic pairs, viz, those originating in 

 the eighth, ninth, and tenth segments respectively. This eighth defini- 

 tive ganglion of the late embryo lies in the anterior part of the seventh 

 somite. 



COELOMIC SACS 



The segmental arrangement of the coelomic sacs characteristic of 

 annelids and arthropods is manifested in the development of flea 

 embryos (pi. 10, fig. yy). The lateral migration of the immigrated 

 cells of the lower layer will be recalled from the account given in a 

 previous section. By this migration a complete layer of mesoderm 

 is formed below the ectoderm. This extends the full width of the 

 germ band. Soon the extreme lateral margins of this lower layer 

 become thickened to form the mesodermal bands. The mesoderm, 

 like the superficial layer, is divided into metameres by the segmenta- 

 tion process. In most of the mesodermal somites formed by this 

 process, there is developed a pair of coelomic sacs (pi. 2, fig. 21 ; 

 pi. 10, fig. 76). These arise as small cavities within the intrasegmental 

 regions of the mesodermal bands. In flea embryos the lumina of these 

 cavities are bounded by thick walls and are similar to those described 

 by Heider (1889) for Hydrophilus. There is no communication 

 between the sacs of adjacent segments to form mesodermal tubes 

 such as Nelson (191 5) found in the embryos of Apis. In regard to 

 the nature of these rudimentary coelomic primordia in fleas it may 

 be stated that it is intermediate between the condition manifested in 

 Apis and that which occurs in many Diptera. In the embryonic 

 development of the Muscidae there is no indication of coelomic sacs 

 according to Graber (1889). Similarly, Gambrell (1933) and Butt 

 (1934) found no coelomic cavities in the embryos of Simidium and 

 Sciara respectively. Moreover, owing to the small size and the thick 

 walls of the coelomic sacs occurring in fleas, these structures in the 

 embryos of the Siphonaptera are very different from their homologs 

 in such primitive insects as Lepisma and the Orthoptera. In all of 

 these forms, Heymons (1895, 1897) discovered that the primitive 

 mesodermal cavities are very extensive and possess thin walls, ap- 

 proximating the type found in such lower arthropods as Peripatus. 

 As in Peripatus, they extend into the appendage rudiments and ulti- 

 mately the appendicular portion of each sac is constricted off, leaving 

 the larger dorsal part to partake in the formation of the definitive 

 body cavity. 



Heider (1889) is of the opinion that in Hydrophilus the coelomic 

 sacs represent the divided original lumen of the tube formed during 



