36 DEVELOPMENT OF THE FROG'S EGG [Ch. IV 



upper hemisphere, yet the phxne of first division is always 

 across the long axis of the egg. Hence, it is fair to assume 

 that tlie segmentation-spindle does so orient itself after the 

 fusion of the male and female pronuclei that lialf of the 

 male and half of the female chromatin are carried apart in 

 the direction of the long axis of the egg., whatever may have 

 been at first the position of apposition of the two pronuclei. I 

 have dwelt on this point at some length because it is one of 

 great importance for our understanding of the relation between 

 egg and embryo ; and because it is much to be desired that the 

 present state of doubt should be cleared away. 



After the protoplasm has divided into two equal parts, the egg 

 " rests " for a time. During the division-period the hemispheres 

 or blastomeres round up to some extent ; but as soon as the 

 division is completed they flatten against each other, so that the 

 cleavage-plane is not so distinctly seen on the surface of the egg. 

 The same process of flattening generally takes place also when 

 the dividing egg is brought into preserving fluids. 



During the time of division we may speak of each blastomere 

 as tending to become itself a sphere, but, owing to the lack of 

 room, the rounding of the two parts is very imperfect. In other 

 eggs (e.g. the eggs of the sea-urchin), where it is possible to 

 remove the egg-membranes, it has been found that then each 

 of the blastomeres approaches more nearl}- the spherical form, 

 or even becomes a complete sphere. We see from this that the 

 external conditions may at least modify the form of cleavage 

 of the egg. 



It is sometimes said that during the division the two new 

 parts or blastomeres tend to repel each other until after the 

 division is completed, and to attract each other after the divi- 

 sion is finished. Such a statement is, however, of little value, and 

 may convey an entirely wrong impression of the changes taking 

 place. One thing seems to be certain, that during the division 

 of the egg the spheres or cells have an influence on one another. 

 Whether unseen protoplasmic connections weld them together, 

 or whether it is merely a question of contact action., has not yet 

 been fully determined. ^ 



1 See Koux's experiments on cytotaxis ('90). 



