NO. 3 EMBRYOLOGY OF FLEAS KESSEL 75 



Fig. 63. Transverse section through the middle portion of the germ band of 

 Ctciwccphalidcs felts where the method of mesoderm formation is 

 similar to that described for figure 62. X 400. 



Fig. 64. Transverse section through the germ band of Ctcnoccphalidcs felis 

 at a point in the posterior extremity of the second-third region, show- 

 ing mesoderm formation by cellular migration following shallow 

 groove formation. The regular course of the mesodermal cells as 

 they move laterally below the ectoderm is also shown. That this 

 stage is somewhat later than that shown in figures 62 and 63 is 

 indicated by the fact that the amnion is formed and that the ectoderm 

 has closed over the point of emigration of the mesodermal cells, 

 thereby practically eliminating the groove. X 400. 



Fig. 65. Posterior portion of a horizontal section through a Ctcnoccphalidcs fclis 

 egg showing the germ band cut transversely at a point just anterior 

 to the region of its involution into the vitellus. This figure illustrates 

 mesodermal differentiation by the formation of a distinct tube and 

 also shows the amniotic cavity in the region where it is about to be 

 transformed into the amnio-proctodaeal cavity. The amnion and 

 serosa are clearly indicated ventral to the amniotic cavity. This 

 section corresponds to that shown in figure 19. X 400. 



Fig. 66. Transverse section through the germ band of Ctcnoccphalidcs felis 

 at a level slightly posterior to that shown in figure 65 and at a 

 somewhat later stage. The mesodermal tube is compressed and its 

 lumen is obscured. The amnio-proctodaeal cavity and the amnion 

 are both clearly shown, and some of the attenuated cells of the 

 serosa may be seen lying against the surface of the vitellus. A 

 region of deutoplasm consequently separates the two embryonic 

 membranes. X 400. 



Plate 9 



Fig. 67. Transverse section through the anterior region of the germ band of 

 Ctcnoccphalidcs felis in that part which is differentiating into the 

 procephalic lobes. The haphazard lateral progress of the meso- 

 dermal cells is shown. The amniotic cavity, amnion, and serosa 

 may be seen above the germ band. X 400. 



Fig. 68. Transverse section through the procephalic lobes of Ctcnoccphalidcs 

 fclis at a stage somewhat later than that shown in figure 67. X 400. 



Fig. 69. Transverse section through the middle region of the germ band of 

 Ctcnoccphalidcs fclis showing the arrangement of the mesodermal 

 cells at the completion of their migration from the blastoderm. The 

 neural groove is beginning to form. X 400. 



Fig. 70. Transverse section through a Ctcnoccphalidcs felis egg showing the 

 germ band cut at two places. The completed amniotic and serosal 

 membranes appear above the upper section. In both sections the 

 neural groove and the layer of mesodermal cells lying below the 

 ectoderm may be seen. X 375- 



