84 PROBLEMS OF FERTILIZATION 



i 



VII. FERTILIZATION AND SYMMETRY 



In 1854 the English naturalist Newport, who had 

 devoted much time to a series of brilliant observations 

 and experiments on the fertilization of the ova of 

 Amphibia, reported "that the first cleft of the yolk 

 is in a line with the point of the egg artificially impreg- 

 nated, and that the head of the young frog is turned to- 

 ward the same point." He thus made the discovery, 

 which has since been confirmed by Roux, Schulze, Bra- 

 chet, and others, that there is a definite relation between 

 the point of penetration of the spermatozoon and the sym- 

 metry of the resulting embryo in the case of the frog. As 

 a result of these studies it has been shown that the pene- 

 tration of the spermatozoon determines that meridianal 

 plane of the polarized egg in which the first plane of 

 cleavage usually forms, and in which the axis of the 

 embryo probably always arises. Subsequent research 

 has also confirmed Newport's statement that the head 

 of the embryo is turned toward the point of penetration. 



Roux attempted the first theoretical explanation 

 of this fact on the basis that the path of the spermat- 

 ozoon determines the plane of apposition of the germ 

 nuclei. The division of the zygote nucleus must/ he 

 thought, run at right angles to the plane of apposition 

 in order to secure impartial distribution of maternal 

 and paternal nuclear constituents to the two cells. 

 Thus the first plane of cleavage would pass along the 

 penetration path of the spermatozoon and approxi- 

 mately through the point of entrance. In certain cases, 

 however, it was shown that the first plane of cleavage 

 did not coincide either with the plane of symmetry or 

 with the fertilization meridian. Brachet's experiments 



