NO. 3 EMBRYOLOGY OF FLEAS KESSEL 3 1 



pi. 9, figs. 6y, 70). The serosa is formed in part by the outer layers of 

 the amnio-serosal folds and in part by the blastodermal covering over 

 the vitellus. In some insect embryos such as those of Leptinotarsa 

 (Wheeler, 1889), Sciara (Butt, 1934), and Brachyrhinus (Butt, 

 1936), a small portion of the dorsal blastoderm does not take part in 

 the formation of the serosa. Instead, the cells of this portion clump 

 together dorsally to form the so-called primary dorsal organ which 

 is soon absorbed by the vitellus. In the embryos of fleas, however, no 

 indication of such a primary dorsal organ is apparent, as the entire 

 dorsal blastoderm is incorporated in the serosa. Soon after com- 

 pletion, the serosa becomes separated from the yolk surface and may 

 for a time be observed as an entirely independent membrane composed 

 of extremely squamous cells lying within the vitelline membrane. 



In contrast to the serosa, the amnion covers only the surface of the 

 germ band, its margins being continuous therewith. The space be- 

 tween this envelope and the embryo constitutes the amniotic cavity. 

 Its anterior portion is formed by the overgrowth of the anterior amnio- 

 serosal fold, its middle part by a similar growth of the lateral amnio- 

 serosal folds (pi. 6, fig. 52), and the posterior part incorporates the 

 lumen produced by the involution of the caudal end of the germ band 

 (pi. 7, figs. 56, 59, 60) . In accordance with the double nature of the 

 posterior portion of the amniotic cavity at this stage, Lassmann ( 1936) 

 has proposed the significant name of amnio-proctodaeal cavity. This 

 term has been adopted here. It is worthy of note that the embryonic 

 rudiment is not terminated at the inner end of this amnio-proctodaeal 

 lumen as might be expected. Instead, it is continued around ventrally 

 for a short distance before it connects with the amnion. 



Histologically, the amnion and the serosa present a similar appear- 

 ance. Both are composed of extremely flattened cells which, in section, 

 appear as thin spindles. At the center of each such cell is a somewhat 

 expanded area containing the nucleus (pi. 2, fig. 22). In the case of 

 the serosa, as this envelope expands in drawing away from the surface 

 of the vitellus, it becomes further attenuated so that its cells become 

 even more compressed. Neither membrane shows any indications of 

 cell division. 



The movements of the germ band in flea development which cor- 

 respond to the blastokinetic processes of hemipteran and odonate 

 embryos are described at this point. Attention has already been called 

 to the fact that the embryonic rudiment in fleas is partly superficial and 

 partly involuted. At the end of the second day of the embryonic period 

 the involution of the caudal region has reached its maximum, and the 

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