POLAR CELLS 257 



nucleus (mi) of the egg, Ascaris being an animal in which the conjuga- 

 tion of ovum and sperm takes place before the maturation of the former. 

 In the nucleus the nuclear membrane, achromatin, and band-like mass 

 of chromatin are visible. The sperm of Ascaris is of peculiar form, and 

 is non-motile. 



B, the same at the commencement of maturation : the nucleus (nn] 

 has travelled to the periphery of the egg and taken on the spindle form. 

 In this and the two next figures the vitelline membrane is shown. 



o 



C, formation of the first polar cell (/. c. i). 



D, the entire egg after the completion of maturation, showing the two 

 polar cells, the first (p. c. i) adhering to the vitelline membrane, the 

 second (/. c. 2) to the surface of the protoplasm : the female pronucleus 

 (pr. nu. ? ) : and the sperm (sp), which has penetrated into the cell- 

 protoplasm, but has not yet become converted into the male pro- 

 nucleus. 



E 1 , E 2 , two stages in the conjugation of the pronuclei in Molluscs 

 (E 1 , Pterotrachea, E' 2 , Phyllirhoe}. 



In E 1 the male (pr. mi. 6 ) and female (pr. nu. 9 ) pronuclei are 

 separated : in E 2 they are applied by their flattened adjacent faces : in 

 connection with each the cell-protoplasm is arranged in the characteristic 

 "sun "-form : the polar cells (/. c.i, p. c.2) are shown. 



F : -F 3 , three stages in the development of the nucleus of the oosperm 

 in a Sea-urchin (Echinus microtuberculatus) : in F 1 the nucleus contains 

 nine chromatin-fibres (chrom. ? ) derived from the female pronucleus, 

 and a globular mass of the same (chrom. 6 ) derived from the male pro- 

 nucleus : the two "suns" are now situated one at each end of the 

 nucleus. In F~ the male chromatin (chrom. S ) has begun to unwind 

 itself : in F 3 there is no longer any distinction between male and female 

 elements, the nucleus containing eighteen similar chromatin-threads. 



G, central portion of the egg of a Hermit-Crab (Eupagurus prideauxii], 

 showing the conjugation of the pronuclei. The male and female chro- 

 matin-networks are fused along the plane of union. The pronuclei are 

 surrounded by finely-granular protoplasm devoid of yolk-spheres. 



(A-F after Boveri ; G after Weismann and Ischikawa. ) 



Maturation consists essentially in a twice-repeated process 

 of cell-division. The nucleus (Fig. 62, A, nu) loses its mem- 

 brane, travels to the surface of the egg, and takes on the 

 form of an ordinary nuclear spindle (B, nu, see p. 63). Next 

 the protoplasm grows out into a small projection or bud into 

 which one end of the spindle projects (c). The usual pro 

 cess of nuclear division then takes place (Fig. 10, p. 64), 

 one of the daughter nuclei remaining in the bud, the other 

 in the ovum itself. Nuclear division is followed as usual by 



s 



