ONTOGENY 



245 



path lies in a plane of a meridian of the egg that bisects the gray 

 crescent. As a result the egg now has a bilateral symmetry. 

 The entrance path also determines the bilaterality of the future 

 embryo, since in the majority of cases the plane of the entrance 

 path coincides with the median plane of the embryo. 



The middle piece of the sperm as it moves through the egg 

 forms a centrosome which soon divides into two. The egg and 



&*&.--, 



^ 







B 



Fig. 151. — A. Unfertilized frog's egg, drawn without the capsule. B. Fertil- 

 ized frog's egg showing gray crescent (c). C. Section of frog's egg in the plane 

 of the first cleavage, showing the bisected gray crescent (c) ; s, penetration path 

 of spermatozoon. D. Surface view of frog's egg during first cleavage. (C and 

 D after Schultze.) 



sperm nucleus come together near the center of the egg; a spindle 

 develops between the centrosomes; chromosomes are formed in 

 each nucleus; the nuclear membranes dissolve and the egg is 

 ready for the first cleavage. The diploid number of chromo- 

 somes of Rana pipiens is 26, of which 13 are furnished by the egg 

 nucleus and 13 by the nucleus derived from the head of the sperm. 

 Mature, unfertilized eggs of the frog can be made to develop 

 by pricking them with a needle. Since under these conditions 

 no additional chromosomes are brought into the egg by a sperm- 

 atozoon, one would expect embryos developing from such eggs 



