138 BERTRAM G. SMITH 



the blastodisc; shreds of non-formative material, such as frag- 

 ments of the nuclear wall and the fibrous material of the germinal 

 vesicle, are each surrounded by a closely adherent film of cyto- 

 plasm and are being absorbed. In eggs ready for fertilization 

 the second polar spindle is sometimes, though perhaps not always, 

 fully formed; it lies beneath a sharply-defined pit at the bottom 

 of which may be found the debris of the first polar body. 



The peripheral zone of fine yolk particles in the vegetal hemi- 

 sphere remains as described in the late ovarian egg. 



C. FERTILIZATION 



The history of the egg-nucleus 



The formation of the egg-nucleus is shown in figures 38 to 40; 

 the process is usually complete about five hours after fertilization. 

 About ten and one-half hours after fertilization (see figs 47 and 

 48) the egg-nucleus has increased in size and sunk into the blasto- 

 disc to a point one-third as far from the surface as the position later 

 occupied by the. copulation-nucleus (see fig. 52). A yolk-free 

 region; partly filled with cytoplasm, extends from the egg-nucleus 

 for a short distance toward the surface, indicating the path of 

 migration (fig. 48) . At this time the egg-nucleus stains but faintly. 



Figs. 41 to 43 Vertical sections of eggs of Cryptobranchus allegheniensis, 

 showing penetration of the egg by the spermatozoon. X 240. 



Fig. 41 From an egg killed 2\ hours after fertilization. This figure is a recon- 

 struction from two adjacent sections : the upper half of the figure is drawn from one 

 section, the lower half from the other. The spermatozoon shown in the figure has 

 entered the egg about 50° from the animal pole where the second polar spindle, 

 shown in fig. 38, is in the late anaphase stage. Another spermatozoon in the same 

 condition as the one figured has entered the opposite side of the egg a little below 

 the equator. 



Fig. 42 From an egg killed 3 hours after fertilization. The spermatozoon 

 figured has entered the egg a little above the equator. This egg contains in all 

 ten spermatozoa. 



Fig. 43 From an egg killed 5 hours after fertilization. The arrow indicates the 

 direction of the path of the spermatozoon which has entered the egg about 30° 

 from the animal pole. The distance from the surface of the egg to the head of the 

 spermatozoon is about one and one half times as great as in the preceding figures. 

 The head of the si)ermatozoon is shown entire in this section; the tail persists in a 

 somewhat abbreviated condition, but is not shown in the section figured. This 

 egg contains another spermatozoon in the same condition. 



