THE INFUSORIA 393 



that the segment containing the meganucleus will regenerate the 

 lost part ; the segment which contains no portion of the mega- 

 nucleus, however, will degenerate and die. Further experiments 

 that have been made on Stentor, on Stylonychia, and other forms 

 prove that a portion of the meganucleus plays an essential part in 

 the regeneration of the segments, and that in all cases the detached 

 parts of an infusorian that are devoid of meganucleus, however 

 large they may be, degenerate and die without repairing their 

 injuries. The nucleated fragments, on the other hand, are capable 

 of regenerating lost parts even when they are exceedingly small, 

 but a limit of size may be reached, which in the case of Stentor is 

 said to be 80 //, in diameter (Lillie), below which even nucleated 

 fragments die. 



There is very little evidence upon the history of the micronuclei in 

 these experiments. Le Dantec, however, states that segments which 

 contain no micronuclei are capable of regenerating lost parts, and that 

 in such cases a new micronucleus is formed by the meganucleus. 



THE MORPHOLOGY OF THE HETEROKARYOTE BODY. 



The generally accepted view that the body of one of these 

 animals is strictly unicellular requires some modification unless our 

 definition of an animal cell is to be widely extended. In the Meta- 

 zoan body we can recognise two classes of cells the somatic cells, 

 which perform the general functions of the body ; and the germinal 

 cells, which are alone concerned with reproduction. We can also 

 recognise two classes of nuclei in the same manner the somatic 

 nuclei and the germinal nuclei. In the Metazoan body there is 

 a large number of somatic cells, and each of them contains, as a 

 general rule, a single somatic nucleus, and similarly each of the 

 many germinal cells contains, usually, a single germinal nucleus. 



Many instances could be quoted (striated muscle, nerve fibres), 

 however, in which the cell outlines of both somatic and germinal 

 cells are ill-defined or absent, so that the tissues become indis- 

 tinguishable from multinuclear plasmodia. There can be no reason, 

 however, for calling such tissues unicellular tissues. The actual 

 number of cells composing the Metazoan body varies enormously, 

 and it is not inconceivable that an animal may have existed 

 (Fig. A) with only one somatic cell and one or two germinal cells, 

 and for protection the germinal cells (m, ra) might be within instead 

 of outside the larger somatic cell (M). 1 



1 Parallel examples of this may be found in the spermatogenesis 

 of Spongilla, Helix, Cossus, etc., in which the germinal cells are within 

 the blastophoral cell. 



