40 EMBRYOLOGY. 



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

 disappearance 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 dis- 

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

 derived from the nucleus of a sperm-cell (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 protoplasm. 

 For the latter is arranged radially 

 around the sperm-nucleus {sk), so 

 that there is formed a radiate 

 figure, which is at first small, but 

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



aftertheoloseofferaUsation, Egg-nucleus u i fi^nretwed and more PX- 



and sperm-nucleus are fused to form the snarpiy expressoQ ana more ex 



cleavage-nucleus (fk), whicli occupies the tended, 



centre of a protoplasmic radiation. -vr • j. x* i 



.Now an interesting phenomenon 

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

 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 (sk), enveloped in its protoplasmic radia- 

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

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

 become surrounded by a common radiation, which now extends 

 through the whole yolk-substance ; are firmly juxtaposed, and then 

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

 each other (fig. 20 fk). The product of their fusion is the first 

 cleavage-nucleus {fk), which undergoes the further alterations leading 

 to cell-division. 



This whole interesting process of fertilisation has consumed in the 

 present object of investigation the short time of about-ten minutes only. 



The phenomena of fertilisation discovered in the Echinoderms were 



