40 EMBRYOLOGY. 



out fluid (probably the nuclear fluid which was diffused after the 



>!>earance of the germinative vesicle). 



The formation of a vitelline membrane is in so far of great signi- 

 ficance for the fertilisation, as it makes the penetration of another 

 male element impossible. No one of the other spermatozoa swing- 

 ing to and fro in the gelatinous envelope is able after that to get 

 into the fertilised egg. 



The one which has penetrated thereupon undergoes a series of 

 changes. The contractile filament ceases to vibrate, and soon di.->- 

 appears ; but out of the head which, as was previously stated, is 

 (Wived from the nucleus of a sperm^cd! (spermatid), and consists of 

 nuclein there is soon developed a very small spheroidal or oval 



corpuscle, which afterwards becomes 

 somewhat larger, the semen- or 

 sperm-nucleus (fig. 18 sk}. This 

 slowly moves deeper into the yolk, 

 whereupon it exerts an influence 

 upon the surrounding protof] 

 For the latter is arranged radially 

 around the sperm-nucleus (/ 

 that there is formed a radiate 

 figure, which is at first small, but 

 Fig. 20. -Egg of a Sea-urchin immediately afterwards becomes more and more 



after the close of fertilisation. Egg-nucleus 11 j j 



and spenn-nucleus are fused to form the Sharply expressed and more 



cleavage-nucleus (/*), which occupies the tended. 



centreof a i>r--r..].l.iMnic radiation. XT 



Now an interesting phenomenon 



begins to hold the attention of the observer (figs. 18, 19, 20). I 

 nucleus and sperm-nucleus mutually attract each other, as it were, 

 and migrate through the yolk toward each other with increasing 

 velocity. The sperm-nucleus (ak), enveloped in its protoplasmic radia- 

 tion, changes place more rapidly than the egg-nucleus (ek). Soon tin- 

 two meet, either in, or at least near, the middle of the egg (fig. 19) ; 

 become surrounded by a common radiation, which now ext 

 thn.n-li t)i. whole yolk-substance; are firmly juxtaposed, and then 

 mutually flattened at the surface of contact; and finally fuse with 

 "ther (fig. 20/fc)- The product of their fusion is the lir,t 

 icleu* (fk\ which undergoes the further alterations leading 

 to cell -divi 



Tlii> whole interesting process of fertilisation has consumed in the 



present object of investigation the. -h <>it timeoi aLoutten minutes only. 



The phenomena of fertilisation discovered in the Echinoderms were 



