CH. II] POLAR BODIES AND FERTILIZATION 23 



the central end the dark streak is rounded, and encloses a clear 

 spot. 



In this clear region one sees a distinct pronucleus about nine 

 microns (/-i) in diameter. Eggs one and a half hours after 

 fertilization show that the pigmented streak has penetrated 

 deeper into the egg, and in the frog the male pronucleus has 

 enlarged to 32 by 22 p (Fig. 11, D, for the toad). 



At this stage another nucleus is present in the frog's egg, 

 and this lies not far from the end of the pigmented streak 

 (Fig. 11, D). This measures 22 /A, and has the same structure 

 as the male nucleus. These two nuclei are undoubtedly the 

 male and female pronuclei. We now know that the female 

 pronucleus has come directly from the original egg-nucleus, 

 which has, after extruding its two polar bodies, penetrated 

 once more deeper into the egg. The complete history has not 

 been traced in the frog, but there can be no reasonable doubt 

 as to what takes place. In the newt (and in the toad) the 

 history has been followed, and it is found that the female pro- 

 nucleus arises from the egg-nucleus after the extrusion of the 

 polar bodies. 



In the next half-hour Hertwig has found that the nuclei 

 approach more nearly to each other, and the pigment-streak 

 penetrates deeper into the egg, the swollen end enlarges, 

 and the two large oval male and female pronuclei are then 

 found together in the swollen end of the streak (Fig. 11, E). 

 In a preparation of an older stage both nuclei have increased 

 in volume to 35 /A, and have flattened against each other. They 

 then fuse into one nucleus which measures 44 p (Fig. 11, F, toad). 

 The resulting nucleus, the segmentation-nucleus, is surrounded 

 by clear protoplasm and then by a pigment-coat. From the 

 segmentation-nucleus a streak of pigment extends to the dark 

 surface of the egg and marks the path of entrance of the 

 spermatozoon. All preparations after two and a half hours 

 showed the union of the two pronuclei. 



If the jelly be examined after the eggs have been laid, 

 several or many spermatozoa can be seen boring their way 

 through the jelly toward the egg. Some will have reached 

 the inner layers, and still others lie in the outer coats (Fig. 

 10). It is probable that after one spermatozoon has succeeded 



