268 Robert Wilhelm Hegner. 



the pole- and ectoderm-cells that one of each taken from Fig, 37 

 has been drawn much enlarged (Fig. 55), The pole-cell is nearly 

 spherical ; its homogeneous cytoplasm stains lightly ; and its nucleus 

 contains besides a nucleolus a number of small groups of cliromo- 

 meres. The ectoderm-cell, on the other hand, is considerably larger; 

 its cytoplasm stains more deeply and contains several vacuoles, and 

 its nucleus, which is without a visible nucleolus, is completely filled 

 with chromomeres. The relations of the parts of the tail-fold are 

 at this stage most easily understood in transverse sections. In 

 Fig. 38 we can still distinctly see the lateral folds (If) of the ventral 

 plate; these enclose the flask-shaped ventral groove (a) within which 

 are the pole-cells luigrating through the pole-cell canal. The amnion 

 which arches over the structures is separated from the dorsal serosa 

 by a thin layer of yolk. The pseudoblastodermic nuclei are still 

 present within the egg. 



The segmentation of the germ-band and the lengthening of the 

 entire embryo now progresses rapidly (Stage K). The cephalic 

 extremity extends completely over the anterior end of the egg, and 

 may be seen covering part of the dorsal surface. The tail-fold has 

 extended a little more than half way up on the dorsal surface of the 

 egg. The segments of the head, thorax and abdomen are visible 

 at this time. The pole-cells have changed their position very little, 

 but a larger number of them are now found inside of the embryo 

 scattered among the mesoderm-cells. A transverse section (Fig. 47) 

 now shows a fairly regular row of pole-cells migrating through the 

 pole-cell canal. The pseudoblastodermic nuclei which have decreased 

 in numl)er by disintegration are represented from this time on only 

 by an occasional nucleus, finally disappearing altogether. 



Our next stage (Stage L) is an important one in the history of 

 the pole-cells. The embryo has become more deeply segmented and 

 its appendages are now evident. The tail-fold which has begun 

 to recede is shown in sagittal section in Fig. 40. In most cases 

 all of the pole-cells have by this time migrated within the embryo, 

 but in the section figured two of them are just entering the pole- 

 cell canal which has become much shortened. A transverse section 

 through the last abdominal segment (Fig. 48) reveals the fact that 



