MATURATION OF THE EGG, AND PROCESS OF FERTILISATION. 39 



are in this respect most excellent objects for investigation. Conse- 

 quently it was by means of them that an accurate insight into the 

 processes of fertilisation- was first secured. They may therefore serve 

 in the following account as the foundation of our description. 



If ripe eggs with egg-nucleus are removed from the ovary into a 

 watch-glass containing sea -water, and a small quantity of seminal 

 fluid is added, a very uniform result is obtained, since in the course 

 of five minutes every one of many hundreds or thousands of eggs is 

 normally fertilised, as can be accurately observed by means of high 

 magnification. 



Although spermatozoa attach themselves to the gelatinous envelope 



Fig. 19. 



Fig. 18. Fertilised egg of a Sea-urchin. 



The head of the spermatozoon which penetrated has been converted into a sperm -nucleus (fr) 



surrounded by a protoplasmic radiation, and has approached the egg-nucleus (ek). 

 Fig. 19. Fertilised egg of a Sea-urchin. 

 The sperm-nucleus (ak) and the egg-nucleus (ek) hare come close to each other, and both are 



surrounded by a protoplasmic radiation. 



of an egg in great numbers, many thousands of them when con- 

 centrated seminal fluid is employed, still only a single one of them 

 is concerned in fertilisation, and that is the one which by the lash- 

 like motion of its filament first approached the egg. Where it strikes 

 the surface of the egg with the point of its head the clear superficial 

 expanse of the egg-protoplasm is at once elevated into a small knob 

 that is often drawn out to a fine point, the so-called receptive promin- 

 ence (Empfdngnisshugel), or cone of attraction. At this place the 

 seminal filament, with pendulous motions of its caudal appendage, 

 bores its way into the egg (fig. 17 A, ). At the same time a fine 

 membrane (fig. 71 C] detaches itself from the yolk over the whole 

 surface, beginning at the cone, and becomes separated irom it by 

 an ever-increasing space. The space probably arises because, in 

 consequence of fertilisation, the egg-plasma contracts and presses- 



