GREAT STEPS IN ORGANIC EVOLUTION 859 



and the "flagellate" ripe spermatozoon are also adaptive in their 

 contrasted characters, is not inconsistent with seeing in these 

 characters alternatives of cell-life which found expression in the 

 primordial organisms. 



In developing as well as in fully formed tissue there are illustra- 

 tions of the deep-seated tendency to three main cellular possibilities 

 — flagellate or ciliated, encysted, and more or less amoeboid, and to 

 a passage from one to the other. In the interpretation of pathological 

 changes the idea of the Cell-cycle seems to be of value. More than 

 that, in the great dichotomies or trichotomies of Organic Evolution, 

 the fundamental alternatives of the Cell-cycle reappear in many 

 a guise. 



BEGINNINGS OF REPRODUCTION —It is instructive to search 

 among the unicellular organisms for the beginnings of reproduction 

 and sex. There are some Protozoa, e.g. Schizogenes, where the 

 division of the cell is not far removed from breakage ; and the giving 

 off of multiple buds in the Rhizopod Arcella is also very primitive. 

 The division of a unicellular organism into two is usually preceded 

 by the division of the nucleus, but in Bacteria that vital centre can 

 hardly be said to have been differentiated. In some Amoebae the 

 nucleus divides in the simple or direct (amitotic) fashion, constricting 

 into the form of a dumb-bell and then breaking across the narrow 

 isthmus; but other Amoebse show the complicated manoeuvres of 

 indirect or karyokinetic (mitotic) nuclear division. Many nuclei 

 may be formed in limited time (one following another in rapid 

 succession) and in limited space (within the wall or the cyst of the 

 parent cell); and this results in spore-formation. 



On another line, leading to fertilisation, one may distinguish 

 many gradations. In plasmodium-forming there is a flowing together 

 of exhausted cells; some cases of multiple conjugation of similar 

 units are known; in ordinary total conjugation there is a union of 

 apparently identical cells (isogamy) as in two Gregarines, or there 

 is the combination of a macro- and a micro-gamete, as in the malaria 

 organism (Plasmodium), or in the common Bell Animalcule. On a 

 special path is the partial conjugation, characteristically complex in 

 Ciliate Infusorians, like Paramoecium, where there is dimorphism 

 of nuclei; and an exchange of micronuclear elements between the 

 two conjugants, after which they separate. 



A very interesting detail is the occasional occurrence of a nuclear 

 reduction before the two unicellulars (or their special gametes) 

 unite, for this is analogous to the nuclear reduction (e.g. polar body 

 forming) in the gametes of multicellular animals. This nuclear 

 reduction is seen in some Ciliate and Flagellate Infusorians, and in 

 some Sporozoa and Heliozoa. When the spherical sluggish macro- 

 gamete in the life-history of the malaria-organism is fertilised, after 



