12 PROBLEMS OF FERTILIZATION 



over twenty years of discussion. The view that sperma- 

 tozoa were parasitic organisms was more or less current 

 until Kolliker in 1841 showed by their development 

 that they were modified cells. Nevertheless, there was, 

 strictly speaking, no immediate application of these 

 results to the problems of fertilization. 



The half-century from 1824 to 1874 yielded relatively 

 little advance in fertilization theory; the opinion that 

 the spermatozoon actually penetrated into the ovum 

 gradually gained ground, largely from the very logic of 

 the situation, but partly from various observations. 

 Bischoff's contact theory, which was the only alternative, 

 was criticized, because if the sperm does not penetrate, 

 but remains outside of the membrane, there is absence of 

 that direct contact between sperm and egg substance 

 postulated by the theory. Wagner's criticism was also 

 very effective; a ferment does not determine the char- 

 acter of a reaction, but the spermatozoon does, for it 

 transmits paternal characteristics. In the way of ob- 

 servations Barry (1840), Newport' (1854-55), Meissner, 

 (1855), and others maintained observations of penetra- 

 tion of the ovum by the spermatozoon; Keber (1854) 

 laid especial emphasis on the micropyle as adapted for 

 entrance of a spermatozoon. These observations were 

 on the whole inconclusive, for actual penetration was 

 not observed but inferred from the presence of spermato- 

 zoa inside the egg membrane. Moreover, the spermato- 

 zoon could not be discovered within the egg. 



' Newport's observations rose to a higher plane than those of the 

 others, for he actually observed in the frog's egg (i) that the first 

 plane of cleavage is in line with the point on the egg artificially impreg- 

 nated, (2) that it marks the plane of symmetry of the embryo, (3) 

 that the head of the young frog is turned toward the same point. 



