266 Robert Wilhelm Hesrner. 



is'- 



a cup-shaped depression in the ventral groove. The ventral plate 

 continues to decrease in length ; this contraction combined with the 

 above mentioned depression, produces a deep cavity at the posterior 

 end of the groove in which the pole-cells lie (Fig. 29). A lateral 

 view of the same egg (Fig. 30) shows part of the pole-cells con- 

 cealed within this cavity. 



The germ-band can now be recognized ; it covers the entire ventral 

 surface of tlie egg except a wedge-shaped area anterior to the groove 

 (Fig. 29). A lateral view of an in toto preparation shows the 

 cephalic region of the germ-band already clearly indicated as a large 

 lateral lobe (Fig. 30). The ventral groove now becomes narrower 

 except at its posterior end ; here a comparatively large opening 

 remains (Fig. 32, «; Stage F), which, since the last stage (Fig. 29), 

 has moved some distance forward. The floor of the groove has at 

 this point invaginated to produce a cavity which extends obliquely 

 upward into the yolk (Stage G). At the bottom of this cavity we 

 find the pole-cells. They now lie entirely below the surface of the 

 egg, partly hidden under the closely opposed lateral folds (Fig. 

 32 If.) 



A sagittal section of an egg in this stage gives a good idea of 

 the structure of the ventral plate (Fig. 33). ISTear the anterior 

 end we can distinguish the beginning of an invagination (a) which 

 will' become the stomodseum. At the posterior end is noticed a much 

 deeper depression which contains the pole-cells lying at the bottom 

 near the entrance of a distinct canal, the "Polzellencanal" of 

 Escherich (1900). This canal is that opening in the blastoderm at 

 the posterior end of the Qgg, which was caused at an earlier period 

 (see preceding chapter. Figs. 23 and 25) by the protrusion of the 

 pole-cells. It can be determined from transverse sections that a 

 posterior depression in the ventral groove is formed by the arching 

 over of the lateral folds, thus producing a flask-shaped cavity (Fig. 

 34, a). The pole-cells lie near the pole-cell canal which contains 

 a mass of cytoplasm connecting them with the pseudoblastodermic 

 nuclei within the egg. 



The next stage of development (Stage H) shows a still narrower 

 germ-band already displaying signs of segmentation. The posterior 



