28 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 98 



them. In contrast, the cells of the future ventral plate, owing to 

 crowding, have elongated and become slightly columnar. The ventral 

 region, therefore, is somewhat thickened. There is, however, no sharp 

 line of demarcation between the thinned and thickened areas (pi. I, 

 fig. 6) . Although somewhat differentiated by cell concentration and a 

 consequential thickening appearing along the ventral surface, the egg 

 is still bounded at all points by a single layer of cells. It is still to be 

 regarded, therefore, as a blastula, and is specifically in the second 

 blastula substage. In fact, another or third blastula substage follows 

 this. 



The transformation from the second to the third blastula substage 

 is accomplished by a division of the cells constituting the thickened 

 areas of the blastoderm. This is the eleventh mitotic division, number- 

 ing from the zygotic nucleus. The cells of the middorsal thinned 

 region do not appear to participate in this division. Owing to the con- 

 centration of the cells in the thickened regions, particularly along the 

 midventral line and its continuation around the poles, some of the 

 nuclei may be forced inward, an illusion of a double cell layer thus 

 being given. A somewhat similar apparently double-layered blasto- 

 derm has been described with different explanations in the eggs of 

 certain other insects including Apis (Kowalevsky, 1871 ; Petrunke- 

 witsch, 1901 ; and Nelson, 1915), Polistcs (Marshall and Dernehl, 

 1905), and Phormia (Auten, 1934). 



The multiplication of cells just described results in a still greater 

 thickening of the germ band anlage. This is enhanced further by a 

 change in shape of the cells of the ventral plate. By this change the 

 cells lose their roughly triangular shape, become elongated, narrowed, 

 and truly columnar. The narrowing allows space for the nuclei to 

 reassume a single linear relationship, restoring the single layer 

 appearance. 



In contrast to the cells of the thickened portion of the blastoderm, 

 those of the thin dorsal region seem to have remained quiescent and 

 from the surface present a squamous epithelial appearance. Another 

 striking difference between the cells of the two regions is that the 

 majority of the columnar cells possess a large vacuole adjacent to 

 the inner margin of the nucleus. This vacuole is absent in the flattened 

 cells. Nelson (191 5) describes a similar stage for Apis, but claims 

 that in the development of this insect the thin dorsal strip regains a 

 thickness equivalent to the remainder of the blastoderm, and then 

 once again is reduced to a thin sheet of flattened cells. No such thick- 

 ening and subsequent rethinning is evident in the eggs of fleas. 



