134 THE PROTOZOA 



larva, it is perfectly comparable to a colony of flagellates such as Volvox, in 

 which the ordinary individuals have lost the power of becoming, or giving rise 

 to, gametes, which can only arise from certain special individuals. 



3. The free-swimming larva, composed mainly of flagellated cells, with 

 the archaeocytes either at the hinder pole or in the internal cavity, undergoes 

 changes as it swims about, which consist in some of the flagellated cells losing 

 their flagcllum, becoming modified in structure, and migrating into the 

 interior of the larva ; in this manner the two germ-layers are established, and 

 the organism has then, so to speak, passed from the condition of a Protozoan 

 colony to that of a true Metazoon. 



4. When the germ-layers are established, the larva fixes itself, and of the 

 subsequent development it is sufficient to state that the cells of the two 

 germ-layers become differentiated into the tissues of the adult sponge, and 

 that in the metamorphosis of the larva the cells undergo a complete rearrange- 

 ment, which shows clearly that every cell has an individuality as distinct 

 as that of any Protozoan individual, a conclusion fully borne out by the 

 recent experiments of Wilson and Huxley (Phil. Trans., B., ccii., pp. IBS- 

 ISO, pi. viii.) on the power of regeneration in sponges after complete separation 

 of the cells from one another. 



I am unable, therefore, to accept the standpoint of Dobell with regard 

 to the relations of Protozoa and Metazoa, but consider that the comparison 

 of a Protozoan individual to a single cell in a Metazoan body is fully justified 

 both morphologically and physiologically, and is a reasonable phylogenetic 

 deduction from the ontogenetic data. The objection that there are no animals 

 known which correspond to the four-cell, eight-cell, and blastula stages in 

 embryological development misses the point and is not strictly true ; the 

 stage at which an embryo consists only of four or eight blastomeres is the 

 homologue of a Protozoan colony, and in the Flagellata species are known 

 in which the colony consists only of four, eight, sixteen, or thirty-two cell- 

 individuals (p. 275). To the query, " Has anyone ever found a metazoon 

 which is composed of nothing but coherent gametes ?" it may be replied 

 that in many Volvocineae the colony also consists only in part of gamete- 

 producing individuals. The theory that the Metazoa arise by cleavage of a 

 multinucleate plasmodium, equivalent to a single Protozoan individual, has 

 often been put forward, but has never found support from a general con- 

 sideration of the facts of Metazoan embryology. In Protozoa the plasmodial 

 phase is always temporary, and ends sooner or later by breaking up into 

 separate uninucleatc individuals. 



4. The Effects of Syngamy (1) upon the Individual, (2) upon 

 the Species. 1. Of all Protozoa, the ciliate Infusoria are the 

 group in which syngamy is most easily observed and studied in 

 the first place because in these organisms it is readily distinguished 

 from simple fission, which is transverse, while in syngamy the two 

 conjugants apply themselves laterally to one another ; in the 

 second place, owing to the fact that the species of Ciliata are 

 practically monomorphlc (p. 440), and can be identified without 

 difficulty. Hence in this group elaborate and exhaustive experi- 

 mental studies upon syngamy and its relation to the life-cycle 

 have been carried out by many investigators, more especially by 

 Maupas, Hertwig, Calkins, and Woodruff. The results of these 

 investigators is briefly as follows : After syngamy the fertilized 

 individuals appear vigorous, feed actively and multiply actively. 

 After many generations of reproduction by fission, however, the 

 race, if kept in an unchanged environment, becomes less vigorous 



