140 BERTRAM G. SMITH 



2. History of the sperm-nucleus 



(a). Penetration of the egg by the spermatozoon. As previously 

 noted, spermatozoa may be found entering an egg taken as early 

 as fifteen minutes after fertilization. In describing the process, 

 we may best begin with an egg taken about two and one-half 

 hours after fertilization (see fig. 41). 



The zona pellucida or vitelline membrane is not affected fur- 

 ther than by the formation of a minute perforation which can 

 only rarely be found in sections. The zona pellucida is omitted 

 in the figures. 



The cell wall of the egg becomes greatly thickened around the 

 perforation made in it by the spermatozoon. The thickened 

 region is conical in form, with the apex of the cone pointing in- 

 ward; its outer and central portions are cross-striated. The per- 

 foration persists as a conspicuous pore lying in the axis of the cone. 

 The entire structure greatly resembles a micropyle. 



Beneath this pseudo-micropyle the path of the spermatozoon 

 is clearly indicated by a yolk-free cytoplasmic region. The form 

 of this region, and the attitude assumed by the spermatozoon 

 itself, indicate that the course pursued by the spermatozoon is a 

 spiral one, with the axis of the spiral lying in a radial direction. 



The spermatozoon at this time retains practically its normal 

 form. As in Axolotl (Fick, '93) and Bufo (King, '01), the tail 

 is not left behind at the surface; in Cryptobranchus it continues 

 to serve as an efficient organ of propulsion. The undulating mem- 

 brane persists, though it is not shown in the figure. The head at 

 this time stains very faintly with the nuclear stain. The acro- 

 some and middle-piece, alwa3''s difficult to see with the magni- 

 fication employed for the study of thick serial sections, have not 

 been observed in this situation. 



Surrounding the shaft of the spermatozoon for a short distance 

 behind the head there is a spindle-shaped yolk-free region contain- 

 ing cytoplasm. This cytoplasm is particularly dense about the 

 region of the middle-piece; from this locality as a center cyto- 

 plasmic strands, resembling linin threads, but finely granular, 

 radiate in all directions, but those extending backward are more 



