SYNGAMY AND SEX IN THE PROTOZOA 133 



stood ; as already pointed out, syngamy has no special relation 

 in Metazoa to cell-multiplication. In Protozoa sexual reproduction 

 means simply reproduction following the sexual act ; but sex and 

 reproduction must be considered as two things entirely distinct. 



The comparison instituted above between the life-cycles of the Protozoa 

 and Metazoa, according to which an entire Protozoan individual is the mor- 

 phological equivalent of a single constituent cell of a Metazoan body, is that 

 which I personally have always held and taught. It is, I believe, the pre- 

 vailing view among zoologists, and has been enunciated clearly by Calkins (5). 

 It has, however, been attacked vigorously by Dobell (110), who lays great 

 stress on the physiological analogy between the single Protozoon. as a com- 

 plete organism, and the entire Metazoan body. On this ground he expresses 

 the view that " a protist is no more homologous with one cell in a metazoon 

 than it is homologous with one organ (e.g., the brain or liver) of the latter " ; 

 he considers it " incredible that anybody could advocate the view that the 

 Metazoa have arisen from aggregated Protozoa," and he puts forward the 

 view that, if the Metazoa have arisen from protist forms, " it is far more 

 natural to suppose that they did so by developing an internal cellular structure, 

 and not by the aggregation of individuals to form a colony." Similar ideas 

 have been put forward also by Awerinzew (890). From these and other 

 considerations, Dobell draws the conclusion that the Protista are not to be 

 regarded as unicellular, but as " non-cellular " organisms. 



So far as the word " cell " is concerned, I have already expressed the 

 opinion above that by the term should be understood a certain stage in the 

 evolution of the Protista, and that many protist organisms should not be 

 termed " cells," but only those which have reached what may be considered 

 as the truly cellular type of organization. I am not, therefore, concerned 

 with Dobell's attack on his own conception of the cell-theory so far as it 

 concerns Protists generally, but only in so far as it applies to the Protozoa. 



It is not possible here to discuss in detail the ontogenetic development of 

 the Metazoa. It must suffice to state that in all primitive types of embryonic 

 development among Metazoa the cells which build up the body originate 

 by repeated binary fission of a single cell, the fertilized ovum ; and that the 

 only cases in which the ovum breaks up into cells by the development of cell- 

 limits internally are those in which the development is modified by the 

 presence of yolk, or where there is good reason to believe that yolk was 

 ancestrally present in the egg. For confirmation of these statements the 

 reader must be referred to the ordinary textbooks of embryology. I must 

 content myself with a single instance, that, namely, with which I am best 

 acquainted by personal study. 



In the development of a simple Ascon sponge, such as Clathrina blanca or 

 other species (see chapter " Sponges" in Lankester's " Treatise on Zoology," 

 part ii., p. 68), the ontogeny may be divided into four phases or periods, 

 which indicate clearly, in my opinion, the general lines in the evolution of 

 the Metazoa from Protozoan ancestors. 



1. Starting with the fertilized ovum, strictly homologous with a Protozoan 

 zygote, it divides by repeated binary fission into a number of cells (blasto- 

 meres), each similar to the ovum in every respect except size ; the process 

 is in every way comparable to the division of a Protozoan zygote into a 

 number of individuals which remain connected to form a colony, as. for 

 example, in many Phytomonadina. 



2. Of the blastomeres thus formed, a certain number, variable in different 

 species, but relatively few, retain their original characteristics, while the rest 

 become differentiated into columnar flagellated cells forming the wall of a 

 cavity (blastoccele). The undifferentiated blastomeres give rise to the 

 archgeocytes. from which ultimately the germ-cells and gametes arise. The 

 flagellated cells are the ancestors of the tissue-cells (histocytes) in the future 

 sponge. At this stage, in which the embryo is hatched out as a free-swimming 



