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William S. Marshall and Paul H. Dernehl, 



is a difference between the blastoderm on part of the ventral surface 

 where the cells become cylindrical, making the blastoderm thicker 

 here than at any other place. Dorsally, the cells become flattened, 

 extending over the sides of the egg and passing by a gradual 

 transition into the ventral cylindrical cells. This thickening of the 

 blastoderm along part of the ventral surface was known before eggs 

 were sectioned. The dorsal cells may become so flat and narrow 

 that they are with difficulty seen. 



In the egg of Polistes a stage occurs in which the blastoderm 

 cells covering most of the surface are of nearly the same shape and 

 size. Fig. 44 is a transverse section of such an egg, cut in a region 

 nearly equidistant from either end and showing all the blastoderm 

 cells to be very similar in outline. An examination of the entire 

 series of this egg gives us near the anterior pole a part of the egg 

 covered with cells which are much more rounded. These shortly 

 give place to others which are different, a section showing a Varia- 

 tion in the shape of the cells surrounding it. At one side of the 

 section the cells are cylindrical and opposite these we find them to 

 be cuboidal; between these two groups the cells are more flattened. 

 Passing further down the egg we soon come to sections in which 

 these variations in the shape of the blastoderm cells give place to 

 the much more regulär appearance seen in the figure. Near the 

 other end of the egg we find the cells again becoming somewhat 

 rounded but not so marked as at the first end observed. We see 

 in this stage, which we hold to be an early one in blastoderm 

 formation, that the cells are not the same over the entire surface of 

 the egg. At and near both poles they are different from the remain- 

 ing surface of the egg, and even the two ends, while covered with 

 cells which are somewhat similar, yet show a marked contrast. 



The section we next figure (Fig. 45) is from an egg which we 

 think is a later stage and which shows the blastoderm cells as 

 having changed their shape over the entire surface of the egg. The 

 cells are not so regulär over any large area as we found them in 

 the preceeding stage, their boundaries being no longer represented 

 by straight, but by curved, lines. No section from this egg will 

 show cells so nearly equal in size and shape as we found in Fig. 44. 

 At one part of the egg, along upper part of drawing, a Space is 

 seen which is without any definite layer of blastoderm cells; nuclei 

 are present at the surface but in a small area no cell boundaries 

 were observed. To select with certainty stages which immediately 



