548 MATURATION AND IMPREGNATION OF THE OVUM. 



Postscript. Two important memoirs have appeared since this paper 

 was in type. One of these by Hertwig, Morphologisches Jahrbuch, Bd. iv, 

 contains a full account with illustrations of what was briefly narrated in his 

 previous paper (13); the other by Calberla, "Der Befruchtungsvorgang 

 beim Ei von Petromyzon Planeri" Zeit. fiir wiss. Zool., Bd. xxx, shews 

 that the superficial layer of the egg is formed by a coating of protoplasm 

 free from yolk- spheres, which at one part is continued inwards as a column, 

 and contains the germinal vesicle. The surface of this column is in contact 

 with a micropyle in the egg-membrane. Impregnation is effected by the 

 entrance of the head of a single spermatozoon (the tail remaining outside) 

 through the micropyle, and then along the column of clear protoplasm to 

 the female pronucleus. 



