FERTILISATION 



33 



poles, near the polar bodies. Here, at the touch of the head of the 

 spermatozoon, a prominence of the outer protoplasmic layer is formed — 

 the receptive prominenee — into which the spermatozoon penetrates. 

 Gradually it presses further into the egg, its tail seeming to fuse with 



B 



c 



Fig. 30.— Fertilisation of eggs of Asterias glaoialis, after Fol (from 0. Hertwig's Lehrhuch 

 der BntwicklUTigsgescliichie). One of the spermatozoa which have entered the mucilaginous 

 envelope comes in contact with the receptive prominence. In C the yolk memhrane is formed. 



the protoplasm of the egg ; the head (the remains of the original 

 nucleus) increases somewhat in size. As male pronucleus, it moves 

 forward to meet the female pronucleus. Finally they fuse and form 

 one single nucleus, the so-called segmentation nucleus. The egg is 

 fertilised. 



It seems tolerably certain that where the egg envelopes have 

 a micropyle, the spermatozoon enters through it. When the first 

 spermatozoon has penetrated the egg, the micropyle is closed by a 

 fresh secretion from the yolk, so that no more spermatozoa can enter. 



Figs. 31 and 32.— Fertilised eggs of a Sea-urcliiii, after O. Hertwlg. Male (sk) and female (et> 

 pronuclei moving towards each other. 



In other cases, at the moment when the first spermatozoon presses 

 in, a membrane begins to rise from the yolk, which makes the entrance 

 of other spermatozoa impossible. There are adaptations also with the 

 object of preventing the entrance into the egg of more than one 

 spermatozoon. 



VOL. I D 



