CH. II] POLAR BODIES AND FERTILIZATION 21 



end of the third or fourth hour, until after cleavage of the yelk 

 has taken place, when it again proceeds, but much more slowly 

 than at first." 1 



In Rana fusca the extrusion of the second polar body takes 

 place one half-hour after fertilization, and the process can be 

 seen under a low magnifying glass or even with the naked 

 eye. A whitish speck appears in the black hemisphere near 

 the point at which the first polar body was extruded. It is 

 necessary, however, to make sections of the egg to discover the 

 further changes that are taking place. Schultze ('87) has 

 given a careful description of the process. The nucleus that 

 remains in the egg after the extrusion of the first polar body 

 assumes once more a horizontal position, but does not go into a 

 resting-stage (Fig. 11, B), i.e. the chromatic loops or threads 

 do not re-fuse into a network nor does a nuclear membrane 

 form. The chromatin arranges itself on a new spindle. The 

 latter then assumes a more or less radial position, and the 

 second polar body is extruded half an hour after the egg is 

 laid. It is probable that the second polar body is not ex- 

 truded under normal conditions until after a spermatozoon has 

 entered the egg. 



One and a half hours after the egg is laid, another change 

 may be seen taking place. Near to or at the apex of the black 

 pole the egg is seen to flatten, and an accumulation of fluid 

 is found here between the egg and its vitelline membrane 

 (Fig. 10). At or near the centre of this flattened portion one 

 may see the fovea, and near or in it the polar bodies appear on 

 the flattened disc. This chamber formed between the flattened 

 egg and the inner membrane was seen by Newport and called 

 the "respiratory chamber." It may ultimately be as large as 

 one-sixth the diameter of the whole egg. Schultze points out 

 that it lies somewhat excentrically with respect to the egg-axis 

 (Fig. 10). The clear fluid in this chamber has been supposed 

 to be the watery contents of the original large nucleus of the 

 egg, which has been squeezed out of the egg. Very little 

 evidence has as yet been given to support this view. Some 

 of the older embryologists thought that this fluid represented 



1 Newport ('51), p. 193. 



