December 21, 1900.] 



SCIENCE. 



943 



exercises no influence upon the power of 

 multiplication which in any wise differs 

 from the influence exerted by any other 

 vital process of the cell. Fertilization and 

 reproduction are phenomena which may be 

 found together, but which in their essence 

 have no connection with one another. 



Leaving the Protozoa, I will now consider 

 the kinds of reproduction in the Metazoa. 

 We formerly supposed that the asexual re- 

 production of Metazoa had been inherited 

 from the Protozoa and that their sexual re- 

 production was a new acquisition. This 

 theory prevailed as long as we thought the 

 Protozoa could onlj' reproduce in an asexual 

 manner. The wide distribution of fission 

 and budding in the lower Metazoa and its 

 entire absence in the Molluscs, Arthropods 

 and Vertebrates, seemed to harmonize with 

 such a view. Although I once held this 

 same opinion, I now consider it incorrect. 

 It seems to me much nearer the truth to 

 make just the opposite statement, viz., that 

 the sexual reproduction of Metazoa is a con- 

 tinuation of the method of reproduction in 

 the Protozoa, while the budding and fission 

 of Metazoa are adjustments having only an 

 outward resemblance to the budding and 

 fission of the Protozoa. 



If we consider the multicellular animal 

 as a cell community, its life history may be 

 resolved into a series of innumerable cell 

 divisions which were preceded by an act of 

 fertilization. This is the same kind of de- 

 velopmental cycle as we find in many Pro- 

 tozoa. For example, in the Gregarines the 

 formation of the pseudo-navicellse and later 

 the sickle-shaped germs follow fertilization. 

 The sickle-shaped germ is comparable with 

 the egg cell, for the Gregarine arising from 

 it suspends multiplication until it has been 

 fertilized. A different character would re- 

 sult in the Metazoa, from the fact that most 

 of the products of division remain united 

 and only certain ones, the sex cells, become 

 self-sustaining. While every cell- division 



in the Protozoa is a similar act of repro- 

 duction, we now distinguish between cell- 

 divisions which bring about the growth of 

 existing individuals and those which permit 

 the creation of new individuals. There is 

 a further difference. The cells which effect 

 the growth and life functions of the multi- 

 cellular organism, the somatic cells of 

 Weismann, have an enormous power of 

 multiplication. The sex cells which are 

 differentiated sooner or later differ from 

 these proliferating cells in that they lose 

 their power of division relatively early. 

 Their characteristic maturation processes 

 are the last expression of this power. The 

 need of fertilization does not necessarily re- 

 sult from multiplication because the sex 

 cells stop multipljang much sooner than the 

 somatic cells which they closely resemble in 

 all other respects. The cell community 

 needs the combination of different kinds of 

 idioplasm and therefore has seized the 

 opportunity which is presented when the 

 organism is in a unicellular state. 



Our conclusion here is similar to that 

 reached in the Protozoa. The occasional 

 mingling of two idioplasms is necessary for 

 the integrity of the cell's life and this is 

 fertilization in the narrower sense. A sec- 

 ond phenomenon may be associated with 

 it, viz., the stimulus to development or re- 

 production. While in the Protozoa fertil- 

 ization is now connected with reproduction 

 and now separated from it, in the Metazoa 

 it is always combined with reproduction. 

 The two occur together as a necessary con- 

 sequence of the multicellular condition, for 

 a mingling of two idioplasms is possible 

 only when the whole organism is contained 

 in a single cell. We have thus fallen into 

 the error of considering fertilization and 

 reproduction inseparable. The recent in- 

 vestigations upon the details of fertilization 

 have caused some of us to break away from 

 this idea, but our opinion has not extended 

 sufiiciently to produce a general conviction 



