NO. 3 EMBRYOLOGY OF FLEAS KESSEL /3 



Fig. 45. Longitudinal section through the posterior region of a Ctenoccphalidcs 

 fclis egg at a stage immediately following that shown in figure 44, 

 showing three germ cells bulging out preparatory to constriction. 

 The granular appearance of the interior of the egg and of a part of 

 the germ-cell cytoplasm is possibly due to Blochmann's corpuscles. 

 X500. 



Fig. 46. Posterior portion of the section shown in figure 42, showing primordial 

 germ cells which have been extruded from the egg mass at the 

 posterior pole. They lie between the periplasm and the vitelline 

 membrane. X 400. 



Fig. 47. Longitudinal section through the posterior region of an egg of 

 Ctenoccphalidcs fclis, also in the third blastema substage of develop- 

 ment. Six germ cells are shown at the posterior pole, lying outside 

 of the periplasm. X 400. 



Fig. 48. Longitudinal section through the posterior region of a Ctenoccphalidcs 

 fclis egg in the first blastula substage of development. The peri- 

 plasm is distinctly divided into cell territories. This section also 

 shows a small group of germ cells which have reentered the egg 

 and are lying just within the blastoderm. X 400. 



Plate 6 



Fig. 49. Longitudinal section through a Ctenoccphalidcs fclis egg in the fourth 

 blastema substage of development. The full quota of first blastula 

 substage nuclei is present but the cell territories characteristic of 

 the blastula stage have not been delimited. X"5- 



Fig. 50. Sagittal section through a Ctenoccphalidcs fclis egg in the second 

 blastula substage showing the crowding of the cells toward the 

 ventral midline which is the first step in the formation of the 

 ventral plate. The thinned dorsal region and the thickened ventral 

 region are clearly distinguishable. X H5- 



Fig. 51. Posterior portion of the section shown in figure 14, showing the 

 flattening of the posterior pole of the germ band, the first step in 

 the formation of the posterior mesenteron rudiment. A group of 

 germ cells may be seen lying inside this flattened region. X 400. 



Fig. 52. Transverse section through the germ band of Ctenoccphalidcs fclis 

 showing the beginning of the formation of the paired lateral amnio- 

 serosal folds. X 400. 



Fig. 53. Sagittal section through the anterior region of a Ctenoccphalidcs fclis 

 egg in a stage slightly more advanced than the one shown in figure 

 23. The anterior amnio-serosal fold and the anterior mesenteron 

 rudiment are both very clearly shown as is also the shallow pit at 

 the point of emigration of the mesenteron rudiment cells. The 

 chorionic and vitelline membranes also show to advantage in this 

 figure. X 400. 



Fig. 54. Sagittal section through the anterior region of a Ctenoccphalidcs fclis 

 egg at a stage somewhat more advanced than that shown in figure 53, 

 showing the double nature of the amnio-serosal fold which has 

 grown farther posteriorly. The ectoderm has entirely closed over the 

 pit which existed above the mesenteron rudiment. X 400. 



