NO. 3 EMBRYOLOGY OF FLEAS KESSEL 29 



The remaining or thickened portion of the blastoderm is termed the 

 ventral plate. As stated, it is thickest along the median line, extending 

 to the anterior pole and somewhat around the posterior pole of the 

 egg. While at first there is no sharp line of demarcation between 

 the thinned dorsal area and the thickened ventral blastoderm, along 

 the midline near anterior and posterior extremities of the ventral plate 

 rather abrupt thickenings soon become evident. These are the first 

 indications of the so-called mesenteron rudiments, which at a subse- 

 quent stage will form the midgut epithelium. They represent the 

 first transformations of the single-cell-layered blastula to a multiple 

 laminated condition. Figure 16 (pi. 2) shows the anterior mesenteron 

 rudiment at the beginning of its formation, figure 15 (pi. 2) at a 

 slightly later stage, and figures 23 (pi. 2), 53, 54 (pi. 6), and 25 (pi. 2) 

 at correspondingly still later stages. The early development of the 

 posterior mesenteron rudiment is indicated in figures 14 (pi. 2) and 51 

 (pi. 6). The details of their later history will be described below. 



FORMATION OF EMBRYONIC MEMBRANES AND INVOLUTION 



OF THE EMBRYO 



The embryos of fleas, like those of most other higher insects, become 

 covered by two embryonic membranes at an early stage in the devel- 

 opment of the germ band. The outer of these envelopes is known as 

 the serosa ; the inner one is termed the amnion. Both are cellular in 

 nature and are produced from that portion of the blastoderm which 

 is not involved in the formation of the ventral plate or embryonic 

 rudiment. The amnion is continuous with the germ band, whereas the 

 serosa, although during its formation continuous in turn with the 

 amnion, ultimately loses even secondary connection with the embryo 

 so as to form an entirely independent covering lying just inside the 

 vitelline membrane. 



The formation of the embryonic membranes in insects, although 

 fundamentally similar, varies considerably in different forms as to 

 its details. These variations are due to the relationship of the germ 

 band to the vitellus. 



The first indication of embryonic membrane formation in flea 

 embryos is the appearance of two shallow indentations of the ventral 

 plate, the beginnings of the amnio-serosal folds. Most authors call 

 these simply the amniotic folds, but because of the dual fates of each, 

 the compound name of amnio-serosal fold used by Wray (1937) has 

 been adopted here. 



One of these indentations is anterior in position, being located at a 

 point on the germ band which is somewhat anterior to the location 



