230 PROCEEDINGS OF THE ACADEMY OF [April, 



true, then in general the earlier the stage of the ontogeny the 

 broader should be the homologies to be determined from it. But 

 this not only does not accord with the facts (as in the case of 

 embryonic structures which could not have been represented in 

 any adult form), but would lead to the absurdity of homologizing 

 all ova —an absurdity because the egg of a mammal is as much a 

 mammal as the adult, and the egg of an arachnid equally well an 

 arachnid. Ova of different organisms are more or less alike, 

 because they all represent single cells with generalized potenti- 

 alities, and hence usually not great structural differentiations; 

 and not because they represent a repetition of a protozoan ances- 

 tor. Sexual reproduction, as clearly shown by Richard Hert- 

 wig, is characterized as reproduction by means of germ cells, 

 whether the germ cell be fertilized or not (parthenogenesis) ; and 

 since sexual reproduction is found in all Metazoa, the beginning of 

 each metazoan individual is a single cell (or a union of two cells). ^^ 

 And the gradual ontogenetic differentiation of the organism being 

 accompanied by cell division, each metazoan must have a two-cell, 

 a four-cell stage, etc. ; in the course of the further differentiation these 

 nuist become arranged into layers, and there must be folding of these 

 layers. Thus agreement in the early ontogeny at least is dii'ectly re- 

 ferable to reproduction by means of germ cells, and to the develop- 

 ment of these cells proceeding by division ; this is the immediate 

 explanation, and much more circuitous is the theory that in the phy- 

 logeny there followed upon the unicellular condition a parvicellular, 

 and upon that a multicellular. Thus the earliest of the processes of 

 the ontogeny, which on the assumption of the biogenetic theory 

 should show the broadest homologies, are the very ones which show 

 homologies least clearly. The ovum of a mammal is a mammal 

 and not a protozoan; and there is no adult protozoan known 

 which is structurally similar to the ovum of any known metazoan- 

 In most cases the cytoplasmic differentiation of a protozoan is much 

 greater than that of an ovum of a metazoan, yet the metazoan is 

 higher in the phyletic scale. 



Thus the organism represents in its ontogeny merely conditions 

 preparatory and subservient to the perfected, terminal stage. The 



" Of course in the case of a protozoan the single cell is at once soma 

 and germ plasm, and is not to be compared to the germ cell alone of 

 Metazoa. 



