GENERAL SKETCH 



137 



1 80, so that the pointed end is directed outward and the middle- 

 piece is turned inward (Fig. 69 A-F), 1 During the rotation a minute 

 aster is developed about the middle-piece as a centre, and at the 



B 



Fig. 68. Fertilization of the egg of the gasteropod Pterotrachea. [BOVERI.] 

 A. The egg-nucleus (E) and sperm-nucleus (S) approaching after formation of the polar 

 bodies; the latter shown above (P.B.); eacli germ-nucleus contains sixteen chromosomes; the 

 sperm-amphiaster fully developed. B. The mitotic figure for the first cleavage nearly established; 

 the nuclear membranrs have disappeared leaving the maternal group of chromosomes above 

 the spindle, the paternal below it. 



1 The first, as far as I know, to observe the rotation of the sperm-head was Flemming in 

 the echinoderm-egg ('81, pp. 17-19). It has since been clearly observed in several other 

 cases, and is probably a phenomenon of very general occurrence. 



