74 LILLIE. [Vol. X. 



In his heteropod work Fol is, if possible, even more explicit. 

 He shows that the region of the shell-gland is at first posterior, 

 and that gradually it comes to lie above the mouth, i.e., 

 dorsally; at the same time the upper pole, as marked by the 

 polar globules, is pushed around anteriorly until finally it lies 

 at the anterior end of the body. The foot area has expanded at 

 the same time. Thus there is a change of axis which has been 

 accompaaied by the formation of new regions, viz. : the shell- 

 gland (dorsal surface), and the foot (ventral surface). Where 

 these two regions meet posteriorly there must be a stationary 

 area, a zone of growth. This region of growth corresponds in 

 position to the first somatoblast of Unio. 



Later authors describe a stationary area in the region indi- 

 cated. Conklin (No. 40) says, " The cells of the posterior arm 

 (of the cross) enlarge greatly and are carried forward until 

 they lie over or even anterior to the cross-furrow, while the 

 point at which the polar bodies are attached (the centre of the 

 cross) is carried forward through an angle of about 90° so that 

 it finally lies at the anterior end of the long axis of the embryo. 

 The position which the polar bodies first occupied (immediately 

 over the cross-furrow) coincides with the middle of the dorsal 

 area, while the ectoderm cells which immediately surround the 

 ectoderm pole are carried forward until they lie at the cephalic 

 pole of the embryo. The endoderm seems to take no part in 

 this shifting, and the ectoderm on the posterior side of the 

 ovum is not shifted forward, but grows around in the opposite 

 direction. There is thus a stationary point in the ectoderm on 

 the posterior side of the ovum in front of which the ectoderm' 

 cells are shoved forward, and back of which they are shoved 

 backward and downward. This stationary point coincides very 

 nearly with what is later the region of the shell-gland." 



This stationary point coincides also very nearly with the 

 region of the first somatoblast. It must be a region of pro- 

 liferation, anteriorly and posteriorly, 



Heymons (No. 47) has witnessed the same phenomena in Um- 

 brella. He says, p. 26 : " Hieran sind lebhafte Wucherungs- 

 processe im Ektoderm betheiligt. Dieselben schliessen sich 

 im wesentlichen an die neuerdings auch von Conklin be- 



