Genu-Cells in Chr}'somelid Beetles. 267 



amnioserosal fold lias grown forward to cover almost half of the 

 embryo, and is mucli further advanced than the anterior fold which 

 has just begun to appear at the lateral edges of the cephalic lobes. 

 The flask-shaped depression in the floor of the ventral groo^'^, which 

 was noticed in the preceding stage (Stage G) has increased in depth, 

 forming the posterior amniotic cavity. The posterior end of the 

 germ-band has forced its way through the yolk in this region, becom- 

 ing a distinct tail-fold (Stage H). 



Sufficient CaUlgraplia niiiterial was not at hand for a study of 

 sections of this stage (Stage H), but eggs of Leptinoiarsa, which pass 

 through a similar course of development, were obtained in great 

 abundance; Figs. 35 and 36 were made from eggs of the latter 

 species. A sagittal section of the tail-fold (Fig. 36) shows five pole- 

 cells situated at the end of the amniotic cavity ; three other pole- 

 cells have already passed through the pole-cell canal, while another 

 has just commenced its journey. This is the first evidence we have 

 found of the migration of the pole-cells from the outside into the 

 embryo. 



A transverse section of another egg {Leptinoiarsa, Fig. 35) shows 

 a similar migration of the pole-cells ; in this preparation a number 

 of pole-cells are lying at the bottom of what was formerly the ventral 

 groove, a few having already entered the pole-cell canal. In both 

 this and the preceding section, the pseudoblastodermic nuclei are 

 fewer in number than in eggs of Calligraplia. 



An embryo (Stage J) slightly older than that just described ex- 

 hibits the following changes: first, the amnioserosal folds have almost 

 completely overgrown the gevm-l)and ; second, segmentation has 

 become more noticeable ; and third, the tail-fold has penetrated still 

 farther forward into the yolk. A sagittal section through the tail- 

 fold (Fig. 3Y) reveals a well developed mesodermal layer just inside 

 of the ectoderm. A number of pole-cells still remain in the posterior 

 amniotic cavity, although more of them have passed through the pole- 

 cell canal than in the embryo last figured (Stage II). The pseudo- 

 blastodermic nuclei which are still present show signs of disintegra- 

 tion. 



Such great differences in structure are now apparent between 



