678 president's address. 



anterior portion, the latter, though devoid of nuclear substance, 

 will form a complete animal. If we accept both of these sets of 

 observations as correct they lead to the important result that, 

 while under normal circumstances regeneration does not take 

 place in the absence of a nucleus, yet, when spontaneous fission is 

 about to take place, something of unknown nature is formed or 

 separated which, though not a nucleus, is capable after artificial 

 fission of giving rise to a new nucleus in the non-nucleated part. 

 This observation of Gruber's in fact throvvs a serious difliculty 

 in the way of the unreserved acceptance of the general principle 

 laid down by him in the earlier of the two papers referred to : — 

 "Zellplasma kann anscheinend immer nur seinesgleichen, nie aber 

 Kernplasma hervorbringen ; das letztere ist mit anderen Worten 

 kein Umwandlungsprodukt des erstern, und damit miisste auch 

 die Moglichkeit freier Kernbildung ausgeschlossen sein." For 

 here appears to be a case in which nuclear substance is formed 

 from cell-protoplasm. 



So far as recorded observations go'it is the chromatin substance, 

 not of the nucleus itself, but of the relatively insignificant para- 

 nucleus, that is of importance in the conjugation of the Infusoria, 

 and it has not been shown that any centrosomes take part in the 

 process. But little of importance seems to have been added to 

 our knowledge of this part of the subject since the date of Maupas' 

 remarkable observations. 



During the maturation of the ovum and the genesis of the 

 sperm-cell it was observed some years ago in the case of Ascaris 

 by Hertwig*, and more recently in the case of a hemipterous 

 insect by Henkingf, that there is a reduction of the number of 

 chromosomes by a half. Though it is diflicult to decide in the 

 present state of our knowledge what deductions to draw from this 

 remarkable fact, the investigations of Gruignard, leading to the 

 result that a precisely corresponding reduction takes place in the 

 case of plants, seem to prove that it has a deep morphological or 

 physiological significance. 



* Arch. f. Mikro. Anat. 1890. 

 + Zeitschr. f. wiss. Zool. 1891. 



