104 GENERAL CONSIDERATIONS. 



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. 39, 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 another, is not a colony in 

 the ordinary acceptation of the term, and it is such a form 

 which, according to what I believe to be the view of Mets- 

 chnikoff and most morphologists, represents the connecting link 

 between the Protozoa and Metazoa. 



But perhaps it will be contended that I am wrong in 

 ascribing this doctrine to them, and that they hold the view 

 that the individuals composing that hypothetical ancestral 

 Metazoon, which is suggested by the cleavage of the ovum, 

 were not completely separate but connected as in living colonies 

 of Protozoa. To this I would reply, that if such be their 

 view, then they can find no justification for it from the de- 

 velopment of forms in which complete cleavage occurs. It is 

 rather in such a segmentation as we find in some Sponges 



