THE NUCLEUS AND CELL-DIVISION. 107 



nuclei by an annular constriction; or there is formed in the interior of the nucleus 

 a fissure, which extends towards the exterior, and so leads to the division 

 of the old nucleus. It is not necessary here to enter further upon other cases of 

 division of the cell-nucleus described by Strasburger and Schmitz; however, the 

 fact discovered by the former, that a fusion into one of previously distinct nuclei 

 occasionally occurs, may be mentioned here. On the other hand, according to the 

 statements of Schmitz, it may also happen in very simply constructed filamentous 

 Algae {C/iroococcacecE, Oscillaricc, Noslocacect,) that a properly defined nucleus is 

 not to be demonstrated at all in the protoplasmic bodies of the small cells. Granules, 

 which are characterised as nuclein by their reactions towards haematoxylin, are 

 found distributed in the whole mass of the protoplasm of the cell. 



When the cell-division runs its course in the typical manner above described, the 

 behaviour of the cell-nucleus easily gives the observer the impression that the impulse 

 to cell-formation proceeds from it ; and that the division of the protoplasmic body and 

 the later origin of the cell-wall are caused by the activity of the nucleus. There 

 are, however, facts which show that, on the one hand, cell-nuclei may be repeatedly 

 divided without a corresponding division of the cell ensuing, as is the case in the 

 large-celled Algae with very many nuclei : on the other hand, again, cell-divisions 

 (i.e. divisions of the protoplasmic body with subsequent formation of partition walls) 

 may also appear in the same plants quite independent of the division of one or 

 several nuclei. The above described tj'pical process of cell-division is thus only 

 the ordinary case, and occurs in higher plants almost without exception. It must 

 not be concluded from this, however, that the described changes of the cell-nucleus 

 are the cause of the division of the cell itself: we have here rather a coincidence 

 of two processes, which in other cases may appear separately — the division of the 

 nucleus, and the cell-division itself. 



There are yet a few other remarks of general importance to be added here. 

 Above all, is to be mentioned the fact that the whole protoplasmic body, with the 

 nuclear substance contained in it, may, under certain circumstances, begin a new 

 life. This, again, happens more frequently in the Algge. The protoplasm &c., 

 becoming loosed from the already existing cell-wall, contracts by driving out 

 water, rounds off, and leaves the cavity of what had hitherto been the cell to 

 swim about for a longer or shorter period as a swarm-spore, in the form of 

 a naked protoplasmic body: finally it becomes fixed, and, after the formation 

 of a new cell-wall, developes into a new plant. Also the case must be mentioned 

 where, by the union of two cells hitherto foreign to one another (or at least com- 

 pletely separate from one another), a single new cell is formed; two protoplasmic 

 bodies containing nuclei fuse together, and represent a single body, which now 

 becomes surrounded with a cell-wall, and sooner or later grows forth into a new 

 plant. If the two fusing cells are of equal size, and generally similar, the process 

 is termed ' conjugation ' : this is the normal form of reproduction in many Algae 

 and sorhe Fungi. Conjugation is, however, only the simplest form of sexual repro- 

 duction, the essence of which consists in that two nucleated masses of protoplasm, 

 which are not in themselves capable of any further development, furnish by their 

 fusion a product which is capable of development. In its typical form, however, 

 this sexual act consists in a relatively large cell, formed only of protoplasm 



