HISTORY OF THE FERTILIZATION PROBLEM 15 



formation that first two or several small nuclei arise, 

 and subsequently fuse; this he finds to occur even in 

 the blastomeres of the four- and eight-cell stages. 



About the same time (1875) Van Beneden also 

 observed similar phenomena in the rabbit's egg. He 

 did not see spermatozoa enter the egg, but he found them 

 with their heads closely applied to the surface in every 

 unsegmented egg, and came to the conclusion that 

 fertilization consisted essentially in fusion of the sper- 

 matic substance with the superficial layer of the vitellus. 

 At a little later stage he found a small nucleus in the 

 cortical layer of the egg; this he called the peripheral 

 pronucleus; a central pronucleus appeared simultane- 

 ously. They grow, approach one another, and meet 

 in the center. Later there is only one nucleus, probably 

 formed by the union of the two. 



As I have shown that the spermatozoa attach to the surface 

 of the vitellus and mix with its superficial layer, it appears probable 

 to me that the superficial pronucleus is formed, partially at least, 

 at the expense of the spermatic substance. If, as I think, the 

 central pronucleus is constituted of elements furnished by the egg, 

 the first nucleus of the embryo would be the result of union of male 

 and female elements. I put forth this latter idea simply as a 

 hypothesis, an interpretation which may or may not be accepted. 



The way was now clear for the definitive solution of 

 the old riddle of the relation of the egg and spermatozoon 

 which was quickly furnished by O. Her twig and Hermann 

 Fol. The observations of these authors appear to have 

 been made independently and nearly simultaneously. 

 In 1875 Hertwig observed and described correctly the 

 principal phenomena of fertilization in the sea urchin 

 egg. He did not actually see the penetration of the 



