FEKTIUSA TION. 



6i 



binations and permutations of nuclear rods and their vital qualities are 

 increased so as to give rise to new variations. 



There are, indeed, other interpretations, and the facts are difficult 

 to understand on any theory. Thus Minot, Balfour, Van Beneden, and 

 others have suggested that the polar bodies are extrusions of male 

 substance from the ovum. Biitschli, Giard, and others interpret the 

 premature division of the ovum as the survival of an ancient habit, and 

 regard the polar bodies as rudimentary or abortive ova. 



It may be possible to combine various interpretations : (l) the ovum 

 divides, like any other cell, like the Protozoon ancestors, at its limit of 

 growth ; (2) the extrusion does in some way differentiate the ovum and 

 renders fertilisation possible or more profitable; (3) the peculiar reduction 

 involved in the process makes the origin of new variations more certain. 



Fertilisation. — In the seventeenth and eighteenth cen- 

 turies, some naturalists, nicknamed " ovists," believed that 



C pb I 



(■*:) fY) 



I 



pb.2 ' 



Fig. 10.- 



-Spermatogenesis and Polar bodies. (After 

 Hertwig and Weismann.) 



Ai. Primitive germ cell o{ Ascci7'is mcgaloccphala var. bivciUjis 

 (4 chromosomes). 



Bi. Sperm mother cell (8 chromosomes). 



Ci. Two spermatocytes formed, each with 4 chromosomes (tirst 

 reducing division). 



Di. Four spermatozoa formed, each with 2 chromosomes (second 

 reducini^ division). 



A. Primitive germ cell (4 chromosomes). 



B. Fully developed ovum (8 chromosomes). . 



C. Formation of first polar hody (pb.T.) (first reducing division). 



D. Formation of second polar body (pl'.i) (second reducing 

 division). First polar body may divide into two. 



the ovum was all-important, only needing the sperm's 

 awakening touch to begin unfolding the miniature model 

 which it contained. Others, nicknamed " animalculists." 

 were equally confident that the sperm was essential, though 

 it required to be fed by the ovum. Even after it was 



