34 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 98 



cavity. The malpighian tubules which serve as excretory organs for 

 the larva have an extremely early origin and arise as invaginations of 

 the proctodaeum (pi. 9, fig. 71). It is possible that they may serve an 

 excretory function in the embryo as well as in the larva. The opening 

 of the proctodaeum into the amniotic cavity is at least significant, 

 especially since in flea embryos the amnion remains intact until the 

 embryonic period is more than half over. When its connections with 

 the embryo are finally broken, the katabolistic products are still pre- 

 vented from contaminating the deutoplasmic food supply by the 

 provisional covering over the vitellus formed by the second dorsal 

 organ. As the latter is absorbed, the amnion forms the third dorsal 

 organ which in turn blocks the exposed surface of the yolk until the 

 dorsal closure of the embryo. This appears to be the first suggested 

 function for either an amniotic or serosal dorsal organ which has been 

 advanced. 



GERM LAYER DIFFERENTIATION 



Coincident with the formation of the embryonic membranes and 

 the involution of the posterior portion of the germ band, there occurs 

 in the development of flea embryos what corresponds to the gastrula- 

 tion process of other animals. In fleas, however, as in other insects, 

 the ontogenetic process is so distorted by heterochrony and coeno- 

 genetic modification that it is difficult to recognize the relationship of 

 this process to the less specialized and more fundamental method of 

 gastrulation by invagination which is so general throughout the ani- 

 mal series. 



The so-called entoderm of insects consists of those embryonic 

 aggregates of cells which go to form the inner lining of the mesen- 

 teron. In flea development, these entoderm anlagen are two in number 

 and consist of the mesenteron rudiments which have been mentioned 

 in an earlier section. The appearance of these rudiments marks the 

 initial step in the transformation of the organism from a simple mono- 

 blastic to a triploblastic stage. Both rudiments appear simultaneously, 

 immediately following the concentration of the blastoderm cells to- 

 ward the ventral surface of the egg. 



The anterior mesenteron rudiment (pi. 2, figs. 15, 16, 23) originates 

 at a point on the future embryonic midline which is ventral to the 

 anterior pole of the egg mass. The first step in the formation of the 

 rudiment is a change in the general appearance of the blastoderm cells 

 over the region where the rudiment is to appear. Previously, all the 

 cells comprising the ventral and lateral thickened areas of the blasto- 



