PRELIMINARY GENERAL SKETCH 



185 



must be carried by the chromosomes ; for their precise equivalence in 

 number, shape, and size is the physical correlative of the fact that 

 the two sexes play, on the whole, equal parts in hereditary transmis- 

 sion. 



2. The Achromatic Structures iu Fertilization 



It is generally agreed that the amphiaster of the primary mitotic 

 figure of the fertilized ovum arises from the egg-substance precisely 



Fig. p2. — Maturation and fertilization of the egg of the mouse. [SOBOTTA.] 

 A. The ovarian egg still surrounded by the follicle-cells and the membrane (z.p. zona pel- 

 lucida) ; the polar spindle formed. B. Egg immediately after entrance of the spermatozoon 

 (sperm-nucleus at g). C. The two germ-nuclei (^, 9) still unequal; polar bodies above. 

 D. Germ-nuclei approaching, of equal size. E. The chromosomes forming. F. The minute 

 cleavage-spindle in the centre ; on either side the paternal and maternal groups of chromosomes. 



as in the ordinary mitosis of tissue-cells, and its mode of origin there- 

 fore involves the same questions as those already discussed at page 72. 

 It is quite otherwise with the centrosomes at the astral centres, the 



