46 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.98 



belongs to the amniotic cavity alone. Its wide continuity with the 

 lumen of the proctodaeum is responsible for the confusion of embry- 

 ologists on this point. None of the five workers who have published 

 on the embryology of fleas considers this relationship, but there is 

 considerable difference of opinion among the authors who have written 

 on the development of other insects possessing a partially involuted 

 germ band. Weismann (1863) and Ritter (1890) both confuse the 

 line of demarcation between the amniotic cavity proper and the 

 proctodaeum. Other investigators working on the muscids, including 

 Graber (1888b), Butschli (1888), Voeltskow (1889), and Noack 

 (1901), have made the same mistake. This error has been in the 

 proper interpretation of that portion of the germ band which lies 

 opposite the main part of the embryonic rudiment and which is con- 

 tinuous with the amniotic membrane. The above authors agree in 

 regarding this short strip as a part of the amnion, and consequently 

 suppose that the amnion forms one of the walls of the proctodaeum. 

 Their identification of the early stage of the proctodaeum is, never- 

 theless, correct. Other workers, however, among whom are Hasper 

 (1911) for Chironomus, Gambrell (1933) for Simiilium, and Butt 

 (1934) for Sciara, have not found the protodaeum in this early stage. 

 For this reason, they all state that the stomodaeum is discernible much 

 earlier than the proctodaeum. 



In the development of flea embryos, the early stages of the procto- 

 daeum are less obscure, owing to the fact that observation is not 

 further complicated by the tendency of the posterior portion of the 

 germ band to roll into a spiral as it is in the dipterans mentioned above. 

 In fleas, therefore, it is possible to follow the development of the 

 proctodaeum from the time of its first appearance. This appearance is 

 simultaneous with that of the stomodaeum and with the involution 

 of the posterior part of the embryo. With the withdrawal of the 

 posterior end of the germ band from the yolk to the dorsal surface 

 of the egg, the proctodaeum already shows a tendency to become 

 directed anteriorly, for at this stage it is perpendicular to the adjacent 

 part of the embryonic rudiment (pi. 10, fig. 75). In respect to the 

 embryo it is now pointing dorsally instead of posteriorly as before. 

 The majority of the posterior mesenteron rudiment cells lie anterior 

 to it (toward the posterior pole of the egg) at this stage. 



The hind-intestine of the late embryo is not a straight tube. It 

 extends anteriorly from the anus to about the anterior margin of the 

 sixth segment and then curves ventrally upon itself and passes pos- 

 teriorly to the region of the eighth segment. Here it again turns 

 sharply ventrally and anteriorly to unite with the mesenteron. The 



