Dbcembbe 21, 1900.] 



SCIENCE. 



945 



are wiped out. It sometimes happens that 

 cells which are in every other way like 

 gametes develop without fertilization, if 

 they are prevented from copulating. This 

 seems to be analogous to the gradation in 

 the need of fertilization which is found in 

 the Protozoa. We are further reminded of 

 ■the Protozoa by the fact that fertilization 

 often leads to the formation of resting 

 spores. 



If we now attempt an accurate statement 

 of the kinds of reproduction in the plant 

 and animal kingdoms, the old conception 

 of sexual and asexual reproduction must 

 be given up entirely and replaced by the 

 following statement. 



All organisms effect their reproduction in 

 a common way by means of single cells 

 which have arisen by cell-division. In 

 single-celled organisms every cell- division 

 is an act of reproduction and results in the 

 formation of another physiologically self- 

 sustaining individual. In multicellular ani- 

 mals, most of the cell divisions lead to the 

 growth of the multicellular individual and 

 only certain of them serve for reprod..ction. 

 Fertilization goes on side by side with re- 

 production, because the organism cannot 

 attain its highest development without the 

 union of two individualities by nuclear cop- 

 ulation. Fertilization in its essence has 

 nothing to do with reproduction. In many 

 single-celled organisms the two occur quite 

 independently and are united for what we 

 call sexual reproduction only under special 

 conditions. Such special conditions are im- 

 posed upon all multicellular animals, since 

 a mixture of two idioplasms could be easily 

 accomplished only during the unicellular 

 stage. Hence fertilization takes place when 

 the single-celled reproductive bodies are 

 formed. It in no wise follows that all 

 such reproductive bodies must be fertilized. 

 One would naturally expect that reproduc- 

 tive cells not needing fertilization (spores) 

 and such as are destined for fertilization 



(gametes, eggs, spermatozoa) should exist 

 side by side. This is the case in plants, 

 though in multicellular animals no genuine 

 case of spore-formation has been demon- 

 strated beyond question. 



The one case which can be pointed to with 

 strong probability is the above mentioned 

 reproduction of the Dicyemidse. Every- 

 where else in the Metazoa spore formation 

 seems entirely supplanted by sexaal repro- 

 duction. All cases of development from 

 single unfertilized eggs are apparently par- 

 thenogenesis and to be explained as sexual 

 reproduction in which fertilization has been 

 lost. The significance of heterogenesis in 

 the di-genetic Trematodes is doubtful. In 

 accordance with the prevailing view, I make 

 a sharp distinction between spore-formation 

 and parthenogenesis resp., heterogenesis, 

 between reproduction by single cells which 

 never have been fertilized and reproduction 

 by cells in which the fertilization has been 

 lost. I doubt if such a distinction could be 

 practically carried out in every case. As 

 long as the reproductive cells are developed 

 in special germinal glands, as for example 

 in the Crustacea, there can be no doubt 

 that we are dealing with parthenogenesis. 

 It is quite different, however, where no egg- 

 glands are differentiated, as in the Eedise 

 and Sporocysts of the Trematodes. In such 

 cases only a more accurate study of the first 

 stages of development will elucidate the 

 matter. In all cases of parthenogenesis 

 which have been carefully investigated the 

 maturation has been preserved. This has 

 hitherto been always regarded as the fore- 

 runner of fertilization. Even in the Pro- 

 tozoa it is connected with the sexual process. 

 Its existence in reproductive cells which de- 

 velop without fertilization therefore favors 

 the view that fertilization formerly did take 

 place. Onthe other hand, one would suppose 

 that spore-formation, like the ordinary divi- 

 sion of the Protozoa, is without polar bodies. 

 Unfortunately we can only speculate upon 



