ONTOGENY 



243 



Egg of the Frog. — The freshly laid egg of Rana pipiens is 

 spherical in form and measures about 2 mm. in diameter; it is 

 bounded by a thin membrane, which in turn is surrounded by the 



PB 



•-^ •--%•-■->--";, .^'/^C :'.' ft 



•SN 



Fig. 150. — Penetration of the spermatozoon in the egg of Nereis, a, same 

 stage as Fig. 149f, showing first maturation spindle; b, 54 minutes after insemina- 

 tion, showing rotation of sperm head and cone, and the origin of the sperm aster 

 from the pole of the nucleus opposite the cone; c, 67 minutes after insemination 

 showing the sperm nucleus between the unequal centrosomes and asters of the 

 sperm amphiaster; d, later stage after insemination showing the fused egg and 

 sperm nucleus lying between the cleavage centers which seem to be a further 

 development of the sperm amphiaster. ec, entrance cone; Mi, M2, first and 

 second maturation spindle; pbi, first polar body; sn, sperm nucleus. (After 

 F. R. Lillie.) 



jellylike egg capsule. More than half of the egg is colored by a 

 dark pigment located in the outer portion of the cytoplasm, 

 leaving a much smaller, cream-colored area on the opposite side. 



