10 



INVERTEBRATA 



CHAP. 



its equatorial plate lies at right angles to both female and male pro- 

 nuclei, and so both are halved at the ensuing division and equal 

 parts of both distributed to the first two cells into which the ovum 



w 



'*$&& 



FIG. 3. Four stages in the maturation and fertilization of the egg of Crepidula plana. 



(After Conklin. ) 



A, formation of first polar body ; the spermatozoon has entered the egg and has begun to swell up 

 into the male pronucleus. B, formation of second polar body ; the first polar body has divided into 

 two. C, the male and female pronuclei have come together. D, formation of the first cleavage spindle ; 

 the female pronucleus above and the male pronucleus below are still clearly distinguishable from one 

 another, fp, Female pronucleus ; mp, male pronucleus ; pi (in A), first polar body, (in B and C), two 

 cells resulting from division of first polar body ; /> 2 , second polar body. 



divides. In some few cases it is possible to distinguish in this first 

 equatorial plate the chromosomes derived from male and female 

 pronuclei respectively, for they are of different sizes and arranged 

 in two different groups. This is especially clear in the egg of Crepi- 



