SIPHONAPTERA AND DIPTERA 359 



the first part of the resulting invagination to form appears to belong to 

 both the proctodaeum and the amniotic cavity and has consequently 

 been termed the " amnioproctodaeal cavity," following Lassmann 

 (1936). Since this first portion of the cavity of the invagination is 

 bounded on both sides by the embryonic rudiment, it is definitely the 

 lumen of the proctodaeum. Later a portion of the extraembryonic 

 blastoderm is drawn into the yolk to form the internal section of the 

 amnion. That part of the cavity which is bounded on one side by the 

 amnion is the only portion of the entire invagination that becomes the 

 amniotic cavity alone. Stomodaeum and proctodaeum appear simul- 

 taneously; the former is nearly straight; the latter soon becomes coiled 

 and in some stages is curved somewhat laterally to the mid-line. The 

 Malpighian tubes arise as diverticula of the blind end of the proctodaeum 

 and are four in number, their blind ends remaining free in the haemocoele. 



The mid-gut lining is derived from the two entodermal rudiments 

 that have already been described. With the shrinkage of the yolk and 

 the invagination of the stomodaeum and proctodaeum, the two ento- 

 dermal rudiments are carried into the interior of the embryo. From each 

 rudiment is proliferated a pair of tongue-like processes which grow toward 

 each other, fuse, and widen gradually, until they form a closed tube 

 enclosing the yolk, in the same manner as has been described for a 

 number of other insects. The dorsal closure of the tube is delayed until 

 the mass of cells composing the ruptured amnion has sunk into the yolk. 

 The breaking down of the cells that form the blind ends of the stomo- 

 daeum and proctodaeum establishes the continuity of the alimentary canal. 



The development of the nervous system of the flea offers nothing 

 particularly striking. Soon after the differentiation of the inner layer, 

 the neural groove appears along the entire mid-ventral line of the 

 germ band, even before the first manifestations of the cephalic append- 

 ages. Instead of originating by invagination, the neural groove appears 

 to be produced by two longitudinal thickenings of ectoderm, one 

 on each side of the mid-line of the germ band, the median unthickened 

 portion becoming the groove. The protocerebral and deutocerebral 

 ganglia form anterior, the tritocerebral ganglion posterior, to the stomo- 

 daeum, the last mentioned later moving forward to fuse with the first to 

 form the definitive brain. A coalescence of the ganglia of the three 

 gnathal segments forms the subesophageal ganglion. Of the 10 abdomi- 

 nal ganglia the last 3 fuse to form one definitive ganglion. 



The most novel thing in connection with the development of the 

 mesoderm of the flea is the presence of coelomic sacs, these being absent 

 in the embryos of the nemocerous Diptera, the nearest relatives of the 

 Siphonaptera. The mesoderm forms as a complete layer the full width 

 of the germ band. This later thickens on the extreme lateral margins 



