of Edinhurgli, Session 1883-84. 
279' 
Thus then it will be sufficiently evident that the morphological 
classification of tissues must be based upon the cell-cycle, the various 
permanent tissues being viewed as specialisations of one or other of 
its fundamental forms, or perhaps sometimes as synthetic types 
between them. And, finally, compressing the gist of several possible 
papers into as many passing allusions, it is evident that the theory 
affords us a basis for the criticism and compression of the recent 
literature — (1) of intercellular digestion (natural to the amoeboid 
phase); (2) of that long dispute respecting the origin of the sexual 
elements of Hydrozoa, from ectoderm to endoderm (the cells of both 
of which show the cycle, and either layer thus develop ova or sper- 
matozoa); (3) of the coelome theory. 
10. Physiological Rationale of the Cell-Cycle. — It is now time to 
demand some physiological rationale for this cycle, which has been 
hitherto regarded as of morphological interest alone. A mass of 
protoplasm anywhere is under constantly varying conditions — at 
one time receiving abundant energy 'from the environment, at 
another little or none. These variations are at least of three main 
kinds — (1) temperature, (2) light, (3) food. Thus, then a rhythm 
of more or less vital activity in definite relation to these conditions 
of the environment is inevitable. 
It is unnecessary to remind the histological reader how often and 
how easily the existence of this rhythm is verified by actual obser- 
vation. Every student is shown the intensification of amoeboid or 
ciliary movement by heat, and its depression by cold or electric 
shock, and knows too the influences of various reagents or gases 
(he., of modification oifood in the general sense) in stimulating or 
retarding activity. The dependence on climate of the cell-cycle of 
the lower organisms, e.y.. Protococcus or Amoeba, is familiar to every 
microscopist. The amoeboid state varies widely with food and 
temperature ; while the actual transition from the ciliated stage to 
the amoeboid, and conversely, have been repeatedly observed ; 
witness the papers of Haeckel, Lankester, and others including the 
writer.* They can only be viewed in fact as distinct from the 
morphological point of view ; physiologically, they show but the 
extremes of one motile state. 
* “On the Morphology and Physiol, of the Cell,” Trans. Roy. Rhys. Soc. 
Edin., 1882. 
