44 NATURAL SCIENCE. Jan.. 



off of the polar bodies as thereby incapable of further division with- 

 out additional chromatin elements derived from the spermatozoon. 

 Boveri, hov^ever, has shown that, in certain cases, the spermatozoon 

 may remain in the periphery of the ovum instead of travelling towards 

 the egg nucleus. The sperm-centrosome, however, does not likewise 

 remain inactive, but approaches the egg nucleus and divides into 

 two. The egg nucleus then divides karyokinetically, and with it the 

 whole body of the ovum. The spermatozoon only comes into activity 

 when the 4- or 8-celled stage has been reached, when it fuses with the 

 nucleus of that blastomere in which it happens to lie. In this case it 

 is obvious that the egg nucleus, even after giving off the polar bodies, 

 has sufficient chromatin left to enable it to divide and reproduce the 

 organism, but at first lacks the organ whereby that division can be 

 brought about. Many facts of this description could be adduced to 

 prove that the presence of a centrosome is necessary for division to 

 take place, and since it has been shown that a spermatozoon can " ferti- 

 lise" a non-nucleate fragment of ova, and develop into larvae of exactly 

 the same species as the animal from which the spermatozoon was 

 taken, the following important conclusion must be drawn as appli- 

 cable to most cases : — A mature ovum and a mature spermatozoon 

 have both sufficient nuclear matter for the production of a new indi- 

 vidual, but cannot do so independently — the ovum because it lacks a 

 centrosome, the spermatozoon because it has not the requisite quantity 

 of protoplasm. The object of fertilisation is therefore that each may 

 supply the wants of the other — the fertilised ovum possessing both a 

 centrosome and a sufficient quantity of protoplasm. The advantage 

 of additional chromatin elements derived from the spermatozoa will 

 be dealt with later. 



As to the nature of the archoplasmic sphere little need be said. 

 It appears to be composed of radiating strands of protoplasm, which, 

 however, do not arise from the centrosome. They do hot probably 

 come at all into contact with it, being separated from it by the so- 

 called " heller hof." How they are formed and what is their 

 meaning is at present very little understood. 



Turning now to a consideration of the probable phylogenetic 

 history of the centrosome, the presence of a macro- and micro-nucleus 

 in many Ciliata at once suggests possible homologies. It is 

 unnecessary to go into the details of the nuclear changes which take 

 place in conjugating Infusoria, so admirably worked out by Maupas, 

 for it will be remembered that the micro-nucleus only is the active 

 and important structure corresponding to the nucleus in the cells 

 of the metazoa, the macro-nucleus merely degenerating, to be formed 

 afresh by division of the reconstructed micro-nucleus. Can the 

 centrosome be homologised with either of these ? This, of course, is 

 very doubtful ; but if any homology is to be drawn, it would appear 

 probable that the centrosome is more nearly related to the macro- 

 nucleus. This may seem improbable, since it is the view of most 



