CHAPTER III 

 EXTERNAL FACTORS 



1. GRAVITATION 



IN the large majority of cases there is no definite relation 

 between the vertical and either the axis of the egg, the planes 

 of its segmentation furrows, or the position of the development 

 of the embryo in it. Thus the eggs of insects are laid with the 

 axis making any angle with the vertical, and the same may be 

 said of Crustacean ova. In such eggs as develop freely in the 

 sea (some Mollusca, for example, Polychaeta, Coelenterata, Cteno- 

 phora) the axis and the planes of segmentation undergo a perpetual 

 change of position, and Oscar Hertwig has shown that in the 

 eggs of Echinoderms there is no necessary fixed relation between 

 the direction of the planes of segmentation and the vertical. In 

 these cases it is clear that the features of development referred 

 to cannot depend upon the force of gravitation. 



There are, however, instances in which it seems possible that 

 the directions of the planes of segmentation bearing as they do 

 a constant relation to the axis of the egg may depend upon 

 gravity, since the axis is normally vertical. It was Emil Pfliiger 

 who in 1883 first brought forward experimental evidence to show 

 that this was indeed the case. 



It is well known that the yolk of the hen's egg always turns 

 over so that the germinal disc is uppermost, and the egg of the 

 Frog, free to rotate inside its jelly membrane, invariably takes up 

 a position with the black pole uppermost, the white pole below. 



This property of the Frog's ovum is exhibited alike by the 

 ovarian, the coelomic, the uterine, and the freshly laid egg, by 

 the living egg and the dead egg, by the whole egg, and by 

 portions that contain both kinds of egg-substance, the yolk and 

 the cytoplasm, as Roux showed by floating eggs or fragments 

 in a medium of the same specific gravity. It is simply due to 

 the fact that in the spherical telolecithal egg the heavier yolk 



