102 BERTRAM G. SMITH 



point of actual or attempted entrance of a spermatozoon is 

 often visible as a minute but sharply-defined pit, barely visible 

 to the naked eye; hence the name 'sperm pit' will be used to 

 designate the precise locality where the spermatozoon enters, 

 though the word 'pit' does not always accurately describe the 

 appearance in preserved material. 



The sperm pits are best studied in material killed in the bichro- 

 mate-acetic-formalin mixture and preserved in formalin. The 

 'pits' are not all alike, but readily fall into the following classes, 

 which probably represent consecutive stages in the penetration of 

 the egg by the spermatozoon (see fig. 8) : 



(a). A simple pit, deep and sharply defined, as observed in 

 living material. 



(b). The pit is surrounded by a very small circular opaque 

 white spot. 



(c). The pit has disappeared, and the white spot remains. This 

 type is most numerous. (Rarely, the pit persists until much 

 later — see fig. 10.) 



(d). The white spot is surrounded and sharply limited by a 

 dark circular line. 



(e). The white spot is surrounded by two concentric circular 

 lines separated by a narrow space which is darker than the general 

 surface of the egg (best shown in fig. 7). 



It is not always possible to tell from surface views whether the 

 spermatozoon has actually entered the egg, but from the study of 



Fig. 7 Equatorial view of an egg of Cryptobranchus allegheniensis, 15 minutes 

 after fertilization, showing a single sperm pit. The lightly stippled area in the 

 upper part of the figure indicates the extent of the blastodisc. 



Fig. 8 Equatorial view of an egg 45 minutes after fertilization, showing numer- 

 ous sperm pits. 



Fig. 9 View of the animal hemisphere of an egg Sf hours after fertilization, 

 showing a sperm area near the edge of the blastodisc. 



Fig. 10 View of the animal hemisphere of an egg SJ hours after fertilization, 

 showing a later stage in the history of the sperm area. The boundary of the sperm 

 area is a trifle too conspicuous in the figure. 



Fig. 11 Equatorial view of an egg 6^ hours after fertilization, showing further 

 extension of the sperm area. 



Fig. 12 View of the animal hemisphere of an egg 7^ hours after fertilization, 

 showing two sperm areas, on opposite sides of the blastodisc. 



All the figures are drawn from preserved material. X 7. 



