NO. 3 EMBRYOLOGY OF FLEAS KESSEL 33 



(Wheeler, 1889) and Calcndra (Wray, 1937) remains intact until the 

 time of hatching. 



In flea embryos, the amnion, after the rupture of the serosa, 

 becomes completely detached from the embryo and forms a complete 

 envelope enclosing the egg contents for a time. However, it remains 

 for only a short time after the absorption of the serosa. Then it, too, 

 ruptures ventrally and contracts dorsally to form the third dorsal 

 organ (pi. 2, fig. 24; pi. 11, fig. 79). This occurs toward the end of 

 the fourth day. The third dorsal organ is absorbed by the vitellus 

 immediately before the dorsal closure of the embryo. The rupture and 

 absorption of the amnion is similar to the fate of this membrane in 

 Leptinotarsa (Wheeler, 1889) and Calendra (Wray, 1937). It differs 

 from what occurs in the Trichoptera and Chironoiuus (Graber, 1888b) 

 in which forms the amnion persists until eclosion. The rupture of 

 both the serosa and amnion in Siphonaptera contrasts with the fates 

 of these membranes in the Lepidoptera (Ganin, 1869) and in the 

 Tenthredinidae (Graber, 1890), in which forms they both persist 

 until hatching. 



Strindberg (1917) alone, of those who have written on the embry- 

 ology of fleas, mentions the fates of the embryonic envelopes. Appar- 

 ently, however, he was handicapped by insufficient material and was 

 therefore unable to determine the complete sequence of events. He 

 notes that both the embryonic membranes cease to be conspicuous at 

 one stage and then speaks of finding their degenerating cells inside the 

 vitellus a little later. Nevertheless, he states that no dorsal organ is 

 formed, thereby indicating that he missed the significant intermediate 

 stages. 



The question of the functional significance of the amniotic cavity 

 to the development of the embryo is one which suggests itself at this 

 point. Wheeler (1889), in discussing this problem for insects in 

 general, regards this cavity as a place for the temporary deposition of 

 excreted matter and therefore as an organ functionally analogous to 

 the allantois of higher animals. He says : "It seems probable that 

 while the inner ends of the ventral plate cells are absorbing and 

 metabolizing the yolk, their outer ends are at the same time giving off 

 into the amniotic cavity a less amount of liquid waste products." This 

 appears to be a logical explanation especially applicable to the develop- 

 ment of flea embryos for which Wheeler's hypothesis may be extended 

 as follows : As already stated, the terminal portion of that part of the 

 amniotic cavity which is formed during the involution of the embryo 

 is connected with the proctodaeum to form the amnio-proctodaeal 



