520 ADAM SEDGWICK. 



There is no animal composed of a mass of separate and similar 

 cells. All the colonial Protozoa consist of a number of cells 

 connected with one another by protoplasmic filaments ; it may 

 be by long contractile filaments^ as in colonial Flagellates 

 and Vorticellse, or it may be by short laterally springing 

 filaments, as in Vol vox; and it is by means of these connecting 

 threads that the individuals of the colony eflPect the little co- 

 ordination of which they are capable. 



Further, from an a priori point of view, it seems highly 

 improbable that such a number of disconnected units could 

 have formed a stage in the evolution of the Metazoa. Is it 

 possible, then, that there has not been any such stage, and that 

 the so-called colonial Protozoon stage in the Metazoon onto- 

 geny is purely secondary, and has been produced by the 

 mechanical requirements of individual development ? Answer- 

 ing this question for the moment in the affirmative, we come 

 to the alternative view, viz. that incomplete cleavage is the 

 more primitive process. This view, though it possesses, accord- 

 ing to our present knowledge, weaker embryological justifica- 

 tion than the first, has a far stronger basis of facts derived 

 from the anatomy of living forms. While amongst the Protozoa 

 there is no counterpart of the fully segmented ovum, there is 

 a comparatively large number of colonial forms in which the 

 individuals are connected by irritable protoplasm, and of mul- 

 tinucleate forms, in which the protoplasm, though more (some 

 ciliated Infusoria) or less (some Rhizopoda) differentiated, is 

 without that definite relation to the various nuclei which is 

 characteristic of the colonial forms and of cells in general. 



Metschnikoff (No. 22, p. 132), in discussing this very ques- 

 tion, contends that the preponderance of complete cleavage, 

 especially in the lower forms, is a strong argument in favour 

 of the colonial Protozoon origin of the Metazoa. Here I differ 

 with him, for in all colonies that we know of the individuals 

 are connected by protoplasmic filaments, which have arisen, not 

 as the result of fusion, but as the result of the incomplete 

 division of the common parent form. A mass of distinct cells, 

 more or less closely applied to one anotljer^ is not a colony in 



