42 NATURAL SCIENCE. Jan., 



which they have apparently conclusively demonstrated the nuclear 

 origin of the centrosome. 



Mr. Moore has been able to observe the actual formation of two 

 distinct centrosomes, by fusion of bodies he calls dictyosomes, which 

 apparently are parts of the achromatic reticular network. If Mr. 

 Moore's conclusions can be applied to other cases, many previous 

 existing ideas as to the nature of the centrosome will have neces- 

 sarily to undergo considerable modification. Meanwhile, his results 

 must be looked upon as forming an isolated exception. Brauer, on 

 the other hand, considers the centrosome to be always present in the 

 nucleus as a permanent body, which may or may not pass through 

 the nuclear membrane preparatory to setting upkaryokinetic division. 

 As will be seen later on, in discussing the possible phylogenetic history 

 of the centrosome, it is probable that the views of Hertwig on the 

 one side, and Boveri on the other, may be equall}^ true for different 

 cases. 



Peculiar interest was aroused when it was shown that the 

 " Mittelstiick," i.e., that part which separates the head and tail of the 

 spermatozoon, was really a centrosome, taken by the spermatozoon 

 into the e^^g as an organ of division, a fact supported by all the more 

 careful researches. Furthermore, Fol has shown in the case of the egg 

 of an echinoderm, that the sperm-centrosome {i.e., the " Mittelstiick") 

 and egg-centrosome take up their position one at either end of the 

 first segmentation nucleus in the region of the future equatorial 

 plane. Each then divides into two, and the halves travel in 

 opposite directions, thus resulting in four centrosomes, one lying at 

 each corner of the somewhat square-shaped nucleus. Each half 

 of the original sperm-centrosome fuses with half of the original egg- 

 centrosome, thus forming two centrosomes, both of which are partly 

 paternal and partly maternal. This done, segmentation commences 

 in the ordinary way. Although the above process takes place in one 

 case at any rate, it is by no means so certain that it occurs in the 

 large majority of instances which have been studied. On the 

 contrary, it has been found that the centrosome of the egg almost 

 invariably degenerates and disappears before the sperm-nucleus 

 comes into contact with that of the ovum, and consequently 

 every centrosome in every cell of the body of the resulting organism 

 is derived from " Mittelstiick " of the spermatozoon. The view that 

 this egg-centrosome is typically a rudimentary organ was first 

 proposed by Boveri, and his conclusions have been verified by 

 Vejdowsky for Rhyncehnis, and Fick in his admirable paper on the 

 maturation and fertilisation of the egg of Axolotl [Ann. Mikr. Anat., 

 Nov., 1893). 



As to the explanation of these facts, it would seem probable that 

 the degeneration of the egg-centrosome takes place in order to prevent 

 parthenogenetic segmentation of the ovum. This supposition may 

 appear absurd to those who look upon the egg nucleus after the giving 



