ELEMENTARY VITAL PHENOMENA 203 



wanders to the surface (Fig. 83, 77), where it divides twice in 

 succession and gives off the polar bodies (Fig. 83, III and IV). 

 In the meantime, the protoplasm of the sperm-cell has fused with 

 the protoplasm of the egg-cell and withdrawn from further ob- 

 servation. The sperm-nucleus, however, has wandered into the 

 middle of the egg. to which place also the egg-nucleus, after giving 

 off the polar bodies, returns from the periphery. The two nuclei 

 now apply themselves to one another, surround themselves with a 

 transparent envelope, and show distinctly two large chromatic 

 loops in each. At the same time, two centrosomes become visible 

 and begin to surround themselves at opposite sides of the nuclei 

 with a circle of rays (Fig. 83, V). In the thread-worm the nuclear 

 substances do not fuse, but the well-known spindle of nuclear 

 division develops, beginning at the two centrosomes, and the 

 spindle-fibres on either side draw to their respective poles one 

 chromatic loop from the egg-nucleus and one from the sperm- 

 nucleus, so that each half of the egg-cell obtains one nuclear com- 

 ponent from the egg and one from the spermatozoon (Fig. 83, VJ\ 

 The fertilisation is thus ended, and at the same time the first 

 division of the ovum is prepared for ; the latter now proceeds in 

 the usual manner, the egg being constricted through the equator 

 of the spindle, while the nuclei in the two halves assume their 

 resting-form. 



As regards individual points, the fertilisation of the egg of the 

 sea-urchin proceeds somewhat differently. The maturation of 

 the ovum is completely ended when the spermatozoon enters. 

 Further, the egg- and the sperm-nuclei fuse completely into a 

 single nucleus before the division into the first two cleavage-cells 

 of the ovum takes place. Fol ('91) supposed that he had made 

 in the further course of the fertilisation -process an observation of 

 special interest, because it appeared to shed some light upon the 

 behaviour of the centrosome. What he saw was the following : 

 With the sperm-cell, a sperm-centrosome enters the ovum, which 

 still possesses, in addition, its own centrosome. After the union of 

 egg-nucleus and sperm-nucleus the two centrosomes come to lie 

 at the two opposite poles of the common nucleus, which is sur- 

 rounded by a simple protoplasmic radiation. Each of the two 

 centrosomes thereupon divides, constricting itself like a dumb-bell 

 into two, each of which wanders across to the other of the opposite 

 side, a phenomenon that was termed by Fol the " quadrille of the 

 centrosomes." Thus, each half of the original egg-centrosome 

 comes into union with one half of the sperm -centrosome and 

 finally fuses with it, so that only two centrosomes are present 

 again at the opposite poles of the nucleus ; each of these two, 

 however, consists in half of the substance of the egg-centrosome 

 and in half of that of the sperm-centrosome. These two centro- 

 somes now form the poles for the following division of the nucleus 



