204 THE CELL 



Our knowledge of the centrosomes was as early as 1834 

 much advanced by the study of the processes of fertilisation. I 

 expressed the opinion (VI. 85) that during fertilisation a cen- 

 trosome was introduced into the egg with the spermatozoon, and 

 that to all appearance it was really the so-called middle portion, or 

 neck, which functions as the attraction centre in the protoplasmic 

 radiation preceding the sperm nucleus. I compared this to " the 

 small quantity of substance present at the end of the nuclear 

 spindle (the polar substance and the centrosome), which, although 

 only stained with difficulty, can yet be distinguished from the 

 protoplasm," and hence I came to the conclusion that if the com- 

 parison is correct, the radiations of the protoplasm, which occur 

 during fertilisation and cell-division, have a common cause in the 

 presence of one and the same substance. 



Richard Hertwig (VI. 84) repeatedly pointed out that the polar 

 substance, the middle portion of the spermatozoon, and the sub- 

 stance of the true nucleoli are similar in composition. Boveri 

 (VI. 7) was of opinion that the spermatozoon carried a pole 

 corpuscle or centrosome with it into the egg. The question was 

 definitely decided by Fol (VII. 14) and Gnignard (VI. 23b), whose 

 important discoveries will be described later on. According to 

 them the nucleus of the egg, as well as that of the spermatozoon, 

 has a centrosome of its own. Whilst the nuclei coalesce, each 

 centrosome splits up into two parts ; half of the one then unites 

 with one half of the other, and. thus the two new centrosomes, 

 which are situated at the ends of the division spindle, are formed. 



In spite of this discovery, one problem still remains unsolved. 

 Are the centrosomes to be regarded as permanent cell organs 

 of the protoplasm, and if so, are they contained in it during rest, 

 only coming into correlation with the nucleus during division ; or 

 are they to be regarded as special elementary portions of the 

 nucleus, such as the nuclear segments, spindle threads, nucleoli, 

 etc. ? In the latter case they must be enclosed during rest in the 

 nucleus itself, ana only come into relation with the protoplasm 

 during division. 



The material for observation, which we have at present, does not 

 suffice for the solution of this question. It is extremely difficult 

 to follow the movements of the centrosomic substance during and 

 after nuclear division as closely as we can observe those of the 

 nuclear substance, for the centrosomes are so excessively small ; 

 and further, it is not always possible to be sure of rendering them 



