326 SCIENCE PROGRESS. 



changed. Thus the nuclear elements among the proto- 

 plastids multiply generally by karyokinesis, like the nuclear 

 elements in metaplastid cells, and when this process is 

 departed from, it varies in both proto- and meta-plastids 

 towards akinesis in a similar way. 



Now, when we consider the great complexity of the 

 karyokinetic process, the long succession of complex 

 structural changes by which it is brought about, and find 

 that the same set of changes are gone through in the same 

 order, during the division of nuclear elements as widely 

 separated in the organic series as that of a Euglypha, and 

 of the somatic cell of a Mammal, we are forced to the con- 

 clusion that the two series of phenomena have originated 

 in common. It is inconceivable that two such long 

 structural metamorphoses, corresponding throughout their 

 whole cycle, should have been evolved independently 

 at different times, and I see no escape from the 

 conclusion that both the proto- and the metaplastid 

 nuclei are structures which have had a common ancestry 

 in the past. Further, as the relation of the nucleus 

 to the cytoplasm of a Euglypha, or other protoplastid, 

 differs in no way from that of the nucleus to the cytoplasm 

 in a metaplastid cell, I accept, because I see no means of 

 controverting it, the proposition that a protoplastid body is 

 the morphological equivalent of a metaplastid cell. 



With respect to the second problem, concerning the 

 relation of the life cycle of the protoplastid to metaplastid 

 development, the ascertained existence of conjugation 

 among both ciliate and flagellate Infusoria is a factor of 

 supreme importance, because it affords a common starting 

 point, from which to compare the life cycle of the one and 

 the individual development of the other. But before it is 

 possible to regard the act of fertilisation in the proto- and 

 meta-plastid in this light, it is necessary to be quite sure that 

 the conjugation in the protoplastid is really a similar process to 

 that occurring between the reproductive cells of higher forms. 



It is well known that conjugation or fertilisation among 

 the higher animals consists not so much in the fusion of 

 two cells as in the fusion of certain cellular constituents, the 



