858 LIFE : OUTLINES OF GENERAL BIOLOGY 



knowledge on the part of its investigators, exalted to the rank of 

 a sub-phylum with several classes, is rather to miss the point, which 

 is merely to indicate the main pathways of evolution. The multipli- 

 cation of pathways, both divergent and parallel, is a more detailed 

 question of pedigree. The general idea is that the pathways trend in 

 three main directions, of which the flageUate, amoeboid, and encysted 

 phases in the life-cycle of Protomyxa or the like, are, as it were, 

 far-off finger-posts. That the Ciliata are far more highly evolved 

 organisms than the Flagellata is not inconsistent with regarding 

 both as in an evolutionary sense Infusorians. 



But what gives fundamental importance to the idea of the cell- 

 cycle is the fact that the three lines — (i) flagellate (and ciliated), 

 (2) amoeboid, (3) encysted — correspond to the three physiological 

 regimes: (i) of lavish expenditure, "living dangerously", relatively 

 high katabolism; (2) preponderant storing, a life of ease, relatively 

 high anabolism; and (3) a balance or compromise between these two 

 extremes. It is interesting to observe that a flagellate spore may 

 become amoeboid as its income and savings increase, that a Sporozoon 

 which gives off amoeboid processes at the beginning of its life soon 

 becomes corticate and without locomotor processes. That this may 

 be adaptive to circumstances is not inconsistent with its being 

 physiologically necessary. 



The general idea of the cell-cycle is corroborated by observing 

 how it fits in some measure for the cells of multicellular animals. 

 For there we find (i) some very active cells, notably ciliated epithe- 

 lium, or, say, the collared flagellate cells of Sponges, the "flame-cells" 

 of Flat- Worms and the "solenocytes" of Annelids; (2) amoeboid cells, 

 like white blood corpuscles, phagocytes, and osteoclasts; and (3) very 

 sluggish cells, like fat-cells and cartilage cells. In certain cases, 

 moreover, the transitions of the cell-cycle are still echoing, for a 

 flagellate endoderm cell in the food-cavity of Hydra may become 

 amoeboid in tackling a small organism that has been ingested; and 

 in a kind of "sore-throat" the ciliated cells lining the windpipe sink 

 down to an amoeboid phase — a far-off recapitulation of a chapter in 

 the life-history of the simple Protomyxa. There is no contractility 

 or external mobility in the fully developed nerve-cell, but it is a very 

 interesting fact that during the development of a fibre the tip probes 

 its way in the primeval amoeboid fashion. 



Other illustrations may be found in the germ-cells. The young 

 ovum is often amoeboid, that of Hydra being a fine example, and 

 young germ-cells in Hydrozoa often migrate in the body for con- 

 siderable distances. The mature ovum, however, often rich in 

 reserves, is a characteristically encysted cell. A typical spermatozoon 

 with its locomotor "tail" is in this wide sense flagellate; the aberrant 

 sluggish spermatozoa of most crustaceans and threadworms are 

 correspondingly of amoeboid phase. That the "encysted" ripe ovum 



