598 PROFESSOR MARCUS HARTOG. 



sions may receive the same morphological explanation, which 

 I will definitely state thus, as I did in 1891 : the most 

 primitive pairing-cells are zoospores, produced by brood- 

 formation (multiple cell-division), and their descendants have 

 to be formed in the same way : tissue-cells can never be 

 directly transformed into gametes. 



The objection has been justly raised that this gives no 

 adequate reasou for the retention of an atavistic process, 

 which could not have survived had there not been some 

 definite physiological good to the race. But a survey of the 

 facts seems to show that cells cannot fuse unless at 

 least one or other of them is fresh from fission, 

 i.e. provided with a young nucleus. 



In most isogamous organisms, if the zoogametes fail to pair 

 within a short time of their liberation, from the absence of 

 members of other broods than their own, they become incap- 

 able of pairing, and either develop directly, go to rest, or die. 



In the isogamous Confervas the zoogametes are usually 

 formed by a cell-division superimposed on those that 

 produce the ordinary non-pairing zoospores: this is indicated 

 by their smaller size and their frequent possession of only 

 half the number of flagella of the others. 



The Rhizopod TrichosphEerium is apocytial: that is, 

 nuclear divisions are not followed by the cleavage of the 

 cytoplasm, so that the organism becomes multinucleate. It 

 exists in two not wholly similar alternating forms, each of 

 which is determined by the resolution of the apocyte into 

 1-nucleate cells, which escape as zoospores. In the First 

 form, produced from the zygote, these are formed directly 

 by resolution, are incapable of pairing, and grow into the 

 Second form. In the Second form the resolution into zoo- 

 spores is immediately preceded by the simultaneous mitotic 

 fusion of all its nuclei, and the zoospores are exogamous 

 gametes. In the Heliozoan Actinophrys two adults 

 approach : before fusion the nuclei divide, and either mate is 

 divided unequally into a large functional gamete and a small 

 abortive one "polar body." In the fungus Basidiobolus, 



