292 EMBRYOLOGY 



After the mixing of the sexual products, numerous spermatozoa 

 approach the egg-cell by a swinging motion of their tails, but only one 

 penetrates, if the egg is normal and capable of life (Fig. 1, k and rri). 

 The point of penetration is known to be a small conical process, the 

 reception eminence (Empfangnis Hugel), which extends from the 

 egg-surface toward the closest spermatozoon. To others, entrance 

 is immediately made impossible by the fact that the egg at once 

 excretes a fine but impenetrable skin, the membrana vitellina, largely 

 as a protection against this. 1 



The internal fertilization process immediately follows the external. 

 Of the three parts which, as is well known, may be distinguished in 

 the spermatozoon, the head, the middle piece, and the contractile 

 terminal filament, the last is thrown off and has no more importance 

 in the process. The head, on the contrary, which was formed from the 

 nucleus of the spermatozoon-forming cell, and which contains the 

 chromatin (Fig. 1, k, and Fig. 2, sk), begins to change into a small 

 round vesicle which I have called the seed or sperm-nucleus, and 

 which by the absorption of juice from the protoplasm begins slowly to 

 increase in size (Figs. 3 and 4, sk). The middle piece (Fig. 1, m) con- 

 tains a tiny cell-organ, the much-studied centrosome (Fig. 2, c), which 

 in spite of its extreme minuteness plays a striking and important 

 role in the division of the nucleus. It moves in front of the sperm- 

 nucleus, and its position in the living cell is easily recognizable, because 

 in its neighborhood, evidently as a result of a stimulus proceeding 

 from it, the protoplasm arranges itself in radial bands in a figure 

 like iron-filings around the pole of a magnet. 



Interesting phenomena begin now, in rapid succession, to fix the 

 eye of the observer. The original nucleus of the egg and the newly intro- 

 duced sperm-nucleus draw mutually together and move with increas- 

 ing rapidity through the yolk toward one another (Figs. 2, 3, and 4). 

 The sperm-nucleus (sk) which is constantly preceded by the radiance 

 with the centrosome (c) included therein, changes its place more 

 quickly than the egg-nucleus. Soon the two meet in the middle of the 

 egg (Fig. 3), where they are inclosed by a common radiance which 

 has now extended over the entire yolk. They lie against one another, 

 becoming flattened on the contact surfaces, and then lose their 

 separation from one another with the formation of a common nuclear 

 sac. Egg- and sperm-nucleus are thus united to form a common egg- 

 nucleus in w r hich the chromatin of the male and female sexual cells is 

 contained. At this point the internal process of fertilization may be 

 looked upon as concluded. 



Two or more nuclei in the egg-cell were already described several 



1 The formation of the Empfangnis Hiigel was first observed by Fol, when, in 

 connection with mv experiments, he made a very thorough study of the fertiliza- 

 tion process in Erhinodcrmata (liecherches sur la ft'condation et le commencement 

 de rh('nog<'nie, Genf, 1879). 



