88 BIOLOGIC.!/, LECTURES. 



2. In what manner do they originate in a given cell ? This 

 ultimately resolves itself into the problem of cell-division 

 {Cytouicry) on the one hand and cell-fusion on the other. 



3. What are the probable steps in the ancestral history, by 

 which these structures came into existence ? This belonsrs to 

 the broad question of Cytogcny as understood in its phylogenetic 

 sense. 



In following out these ciuestions more in detail, it is 

 important to bear in mind at the outset, some vital distinctions 

 in\-olved in the use of the term organ, whether we understand 

 it in a ^wxo^y pJiysiological, or in a morphological •><ix\'~,<t. P^^om 

 the purely physiological standpoint, any structure or member 

 which, by its activity, contributes to the general welfare of the 

 whole organism is an organ, whether that structure may have 

 originated primarily in the organism, or may have been derived 

 secandarily from an external source. Thus, the chromatophores ^ 

 in the leaves of a plant are the organs of assimilation in that 

 plant ; the " gonidia " in the thallus of a lichen are the organs 

 of a lichen, in a physiological sense as the heart or lung is the 

 organ in the body of a higher organism. 



But from the morphological side of the case it is different. 

 According to the morphological view, every diffcroitiatcd 

 organism, every organism composed of organs, can only Jiave 

 originated from a //omogeneons sfage by the differentiation of 

 its parts. To state this in another wa\', "however complicated 

 one ot the higher animals and j^lants may be, it begins its 

 separate existence under the form of a nucleated cell. This, 

 bv division, becomes converted into an aggregate of nucleated 

 cells : the ])arts of this aggregate, following different laws of 

 growth and multiplication, give rise to the ludiments of the 

 organs ; and the parts of these rudiments again take those 

 modes of growth and multiplication and metamorphosis which 

 are needful to con\ert the rudiment into the perfect structure." ^ 



' TliL' term chromatophorc (Schmitz) is here usud in a broad sense, including 

 leiicoplasts and the various coloring substances in the llower and the fruit, as well 

 as the chrophyll granules in the leaves of a green plant. The term is synonymous 

 with .\rthur Meyer's tro/>/io/>/ast a.x\d. Schimper's/A/.f/'/V. 



- T. II. Huxley : Article /y/otoxy, Encyclopaedia I'.ritannica, 9th edition. \'()1. Ill, 

 p. 6S2, 1S7S. 



