EMBRYOLOGY OF CRYPTOBRANCHUS 137 



Out of twenty-one eggs from three females (C, D and E), 

 preserved at intervals extending from fifteen minutes to two and 

 one-half hours after fertilization, a second polar spindle was found 

 in eighteen cases, and one or more spermatozoa were found in 

 each of eleven eggs. The sections show that the spermatozoon may 

 pierce the cell wall of the egg as early as fifteen minutes after con- 

 tact with the outer envelopes, though a longer time is usually 

 required. 



Making allowance for faults of technique we may say that the 

 second polar spindle is usually and probably always present from 

 the time of fertilization up to two and one-half hours later, reckoned 

 from the moment of mixing the sexual elements ; there is no essen- 

 tial difference in this respect between eggs artificially and naturally 

 fertilized. 



Only early stages of the second polar spindle are found in eggs 

 up to and including one and one-half hours after fertilization; 

 exclusively anaphase stages are found in eggs taken one and 

 three-quarters to two hours after fertilization; the formation of 

 the second polar body and the egg-nucleus (see figs. 39 and 40) 

 is confined to a period between 4 and 8 hours after fertilization. 

 While a stimulus from the spermatozoon is not required to initiate 

 the formation of the second polar spindle, it is evident that the 

 later stages of this mitosis are passed through only after fertiliza- 

 tion ; in other words, the processes of maturation and fertilization 

 overlap. Hertwig ('06) makes the general statement that in na- 

 ture the time of fertilization of the amphibian egg falls between 

 the formation of the first and second polar spindles. 



4. The organization of the egg immediately before fertilization 



At the time of spawning the egg is surrounded by the unchanged 

 zona pellucida or vitelline membrane; within this is a thin cell 

 wall, the transformed zona radiata, which is organically connected 

 with the egg. 



There are few changes in the general appearance of the blasto- 

 disc since the condition described shortly after the rupture of the 

 germinal vesicle (see fig. 31). There is a more intimate incorpor- 

 ation of the materials of the germinal vesicle into the substance of 



