48 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 98 



are the mesenteron ribbons. The ribbons of each side gradually 

 approach one another and fuse, forming a complete band of entoderm 

 connecting the stomodaeum and proctodaeum (pi. 11, fig. 80). The 

 two bands formed in this manner then widen gradually both dorsally 

 and ventrally. Their ventral growth is much more rapid than their 

 dorsal expansion, and as their initial position is somewhat ventral, 

 they meet and fuse ventrally first so that the closure of the ventral 

 wall of the midintestine occurs considerably earlier than its dorsal 

 closure. The dorsal closure is delayed until the third dorsal organ, 

 composed of the massed cells of the ruptured amnion, has sunk into 

 the vitellus (pi. 2, fig. 24 ; pi. 11, fig. 79). Then the dorsal margins 

 of the widened mesenteron ribbons fuse along the dorsal midline and 

 the epithelial lining of the midintestine is completed. 



The method of the enclosure of the vitellus by the cells of the 

 entoderm as here described for flea embryos corresponds essentially 

 to that which has been described for all other insects thus far studied, 

 possessing bipolar entodermal rudiments, except Apis. In the honey- 

 bee, by contrast, and according to the observations of both Grassi 

 (1884) and Nelson (1915), the two mesenteron rudiments each form 

 a median dorsal ribbon instead of a pair of ventrolateral ones. This 

 results in the dorsal surface of the yolk being covered first. 



The final steps in the embryological development of the intestinal 

 tract of fleas are the breaking through of the blind ends of the 

 stomodaeum and proctodaeum to make the lumen of the digestive 

 tube continuous from the mouth to the anus, and the investiture of 

 the entire tract with its muscular layer. 



NERVOUS SYSTEM 



Very soon after the separation of the lower layer cells from the 

 ectoderm, the neural groove appears along the entire midventral line 

 of the embryo (pi. 9, figs. 69, 70). This is the first step in the devel- 

 opment of the nervous system, all of which is produced by the ecto- 

 derm. The neural groove makes its appearance even before the first 

 manifestations of the cephalic appendages. Instead of originating by 

 invagination, the neural groove appears to be produced by two longi- 

 tudinal thickenings of the ectoderm, one on each side of the midline 

 of the germ band (pi. 10, fig. 78), the median unthickened portion 

 becoming the groove. The two ridges continue the full length of 

 the germ band, one passing laterally on each side of the depression 

 which marks the position of the stomodaeal invagination. They are 

 continued, therefore, on the cephalic segments and unite in the head 



