EMBRYOLOGY. 



out fluid (probably the nuclear fluid which was diffused 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 j 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 

 Pig. 20.-Egg of a Sea-urehin immediately afterwards becomes more and more 



afterthe close of fertilisation Egg-nucleus fa , expressed and more 6X- 

 and sperm-nucleus are fused to form the .f.<? 



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



centre of a protoplasmic radiation. 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 fir*t 

 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 



