124 



COMPARATIVE ANATOMY 



spermatozoon. The mouth of the funnel is then filled by a 

 plug of coagulated material known as the entrance-cone. A 



CO 



Fig. 26. 



Fertilisation of the amphibian ovum (after Jenkinson). 



A, outline drawing of a section parallel to the axis of the egg ; the superficial 

 pigment of the animal hemisphere of the egg is indicated, but the yolk 

 granules are omitted, co, entrance cone ; spz, spermatozoon lying at 

 the bottom of the entrance funnel ; s.sfi, spermsphere. , a. meridional 

 section through the egg at a later stage ; Q > sperm-nucleus, also called 

 the male pro-nucleus ; O. , egg-nucleus, also called the female pro- 

 nucleus ; as, sperm-aster ; pb, polar body. The size of the sperm- and 

 egg-nuclei has been exaggerated. C, portion of a section through r>n 

 egg showing an early stage in the formation of the fertilisation spindle, 

 highly magnified ; Q , sperm-nucleus ; *j? , egg-nucleus ; cs, centro- 

 somes. Z>, portion of a section of an egg showing the early stage of 

 the metaphase of the fertilisation spindle ; chr, the chromosomes derived 

 from the sperm- and egg-nuclei lying unevenly, but still in two distinct 

 groups, in the equatorial plane. 



clear space is formed round the head and middle piece of the 

 spermatozoon, the tail is absorbed, and soon the middle piece 

 disappears also, and as it does, the clear space assumes a 



