$2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 98 



posterior third of the embryo lies completely surrounded by yolk and 

 directed toward the anterior end of the egg (pi. 7, fig. 56). During 

 the next 12 hours, however, there is a withdrawal of the involuted 

 portion from the deutoplasm (pi. 7, fig. 55). This process corresponds 

 to the more complex phase of blastokinesis known as revolution, which 

 occurs in certain other insects, but the details are quite different. In 

 the case of fleas it involves no rupture of the embryonic membranes. 

 Furthermore, instead of withdrawing along the line of involution, the 

 involuted caudal portion of the germ band in fleas merely pulls through 

 the yolk toward the dorsal surface of the egg so that when the process 

 is completed the entire embryo lies at the surface (pi. 10, figs. 73, 74, 

 75). During the processes of involution and that which corresponds 

 to revolution of other forms, there has occurred a considerable in- 

 crease in the length of the anterior end of the germ band. At this 

 stage, therefore, it extends entirely around the anterior pole of the egg 

 and is directed posteriorly on the anterior dorsal surface. With the 

 withdrawal of the posterior end of the embryo from the yolk, its 

 anterior and posterior extremities almost touch. The embryo at this 

 stage is about 60 hours old. 



FATES AND SIGNIFICANCE OF THE EMBRYONIC ENVELOPES 



The embryonic envelopes of insects exhibit important differences 

 as to their fates. However, the development of fleas includes a combi- 

 nation of fates for the amnion and serosa which appears to be unique. 



Mention has been made of the fact that no structure, homologous 

 to the primary dorsal organ of such forms as Leptinotarsa, Sciara, 

 and BrachyrJiiniis is found in flea development. Likewise, there seems 

 to be no primary dorsal organ of the type, described by Wray (1937) 

 for Calcndra, which has nothing to do with the embryonic membranes. 

 Nevertheless, in order to avoid confusion with these two types of 

 dorsal structures, the use of the term primary dorsal organ for the 

 first dorsal body formed in fleas is avoided. Therefore, the first such 

 structure differentiated in siphonapteran development is herein called 

 the second dorsal organ. 



In the embryology, of fleas the serosa ruptures ventrally early in 

 the fourth day, and is drawn dorsally to form an indistinct clump of 

 cells which lies for a short time on the dorsal surface of the yolk as 

 the second dorsal organ. It is gradually absorbed by the vitellus. This 

 rupture of the serosa and its formation of the second dorsal organ 

 corresponds to the fate of this membrane in the Trichoptera and 

 Cliironomns (Graber, 1888b). By contrast, the serosa in Leptinotarsa 



