348 EDWIN G. CONKLIN 



ing the period of maturation, preceding or accompanying fer- 

 tilization, two minute cells, the polar bodies, are formed at the 

 animal pole of the egg, and in all animals from sponges to mam- 

 mals the animal pole gives rise to the ectoderm and the vegetal 

 pole to the endoderm of the embryo. 



But while the polaidty of the embryo as a whole corresponds 

 directly to the polarity of the egg from which it develops the axes 

 of the constituent cells of the embryo are shifted in successive 

 cell generations so as to form all possible angles with the chief 

 axis of the embryo. The axis of a cell is usually marked out 

 by the line passing through the center of the resting nucleus and 

 centrosome, this being the 'cell axis' (Van Benedan '83, Heiden- 

 hain '94). In cleavage cells the resting centrosome, or centro- 

 sphere, comes to lie at the free border of the cell at a point as 

 near as possible to the animal pole, consequently in cells lying 

 near the animal pole the cell axis is nearly parallel with the chief 

 axis of the egg, but in cells which lie near the equator of the egg 

 the cell axis may be nearly at right angles to the egg axis and in 

 cells which lie near the vegetal pole the cell axis may be nearly 

 the reverse of the egg axis. As long as the cleavage cells are rela- 

 tively large it is possible to see that the resting centrosomes always 

 lie at that point of the free surface of the cell which is nearest the 

 animal pole, but when these cells become very small it is no longer 

 possible to see that the centrosomes are turned toward the ani- 

 mal pole although they always He at the free border of the cell. 

 Similarly in tissue cells it can be seen in many cases that the cen- 

 trosomes lie at the free border of the cell (Heidenhain and Cohn 

 '97), but there is no evidence of their approximation to the origi- 

 nal animal pole. The same is true of the various stages in the 

 formation of the germ cells, whether ova or spermatozoa, the 

 centrosomes always he on the side of the nucleus toward the free 

 border of the cell and away from its attached side, but no ap- 

 proximation of the centrosome to the original animal pole can be 

 traced. 



The resting nucleus shows polar differentiation, as was pointed 

 out long ago by Rabl ('85), the pole which lies nearest the cen- 

 trosphere being known as the 'Pol' or central pole, the opposite 



