50 



INVERTEBRATE MORPHOLOGY. 



spermatozoon, as a rule, in healthy ova, penetrating and tak- 

 ing part in the conjugation, though apparently in some cases 

 polyspermy, or the penetration of several spermatozoa, may 

 occur. The head of the spermatozoon comes into contact 

 with the protoplasm of the ovum, which in some cases rises 

 up to meet it, and is rapidly engulfed. The tail likewise of 



. oc 



mp 



FIG. 23. DIAGRAMS TO ILLUSTRATE THE PHENOMENA OF FERTILIZATION. 



A, the approximation of the male and female nuclei. 



B, division of the ceutrosomes. 



C, rotation of the centrosomes. 



D, fusion of the ceutrosomes aud nuclei, arid formation of the segmentation 



spindle. 



cc = compound ceutrosome. oc ovum centrosome. 



fp = female nucleus. sc = sperm centrosome. 



mp = male nucleus. sn = segmentation nucleus. 



the spermatozoon is taken into the ovum and seems to be com- 

 pletely absorbed, the head alone being visible in later stages ; 

 it constitutes the male pronucleus (Fig. 23, mp) and moves 

 towards the centre of the egg until it comes into contact with 

 the female pronucleus, without, however, fusing with it. A 

 spindle now makes its appearance, and the two pronuclei p;iss 

 through the various karyokiuetic stages, forming equatorial 

 plates each with half the typical number of chromosomes, 



