GENERAL SKETCH 



m 



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

 piece is turned inward (Fig. 69 A-F)} 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 Pterotracbea. [BOVERI.] 

 A. Tlie egCT-nucleus {E) and sperm-nucleus (5) approaching after formation of the polar 

 bodies; the latter shown above {P.B.)\ each germ-nucleus contains sixteen chromosomes; the 

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

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

 the spindle, the paternal below it. 



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

 the echinoderm-egg ('8i, pp. 17-19). It has since been clearly observed in several other 

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



