COMPLETE AND INCOMPLETE CLEAVAGE. 103 



probable that the connections between the various adult cells 

 of other Arthropoda with centrolecithal eggs will be found to 

 be derived continuously from the connections between the cells 

 of the segnaenting ovum. 



But while we must admit that there are cases in which the 

 cleavage is not complete, yet in a great many animals — in all 

 cases of small holoblastic eggs — it seems to be quite certain 

 that the cleavage is complete. It is true that the spheres 

 always touch one another, and there may be an organic con- 

 nection at the point of contact ; but assuming that there is no 

 such connection, the question naturally arises : which of these 

 two processes — the incomplete or the complete cleavage — is, 

 from a phylogenetic point of view, the more correct ? 



It has generally been supposed that the complete cleavage is 

 the most primitive process, and that the mass of organically 

 distinct and similar cells, such as is found in the morula of a 

 typical development, represents a colonial Protozoon-like 

 ancestor of the whole of the Metazoa. In short, the general 

 view seems to be that the immediate ancestor of the first 

 Metazoon was a multicellular Protozoon, the separate cells of 

 ■which were all distinct from one another. Can we find any 

 justification in the animal kingdom, as we know it, for this 

 view ? Is there any living form constituted in this manner ? 

 The answer is, it is hardly necessary to say, a negative one. 

 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 Vorticellae, or it may be by short laterally springing 

 filaments, as in V o 1 v o x ; and it is by means of these connecting 

 threads that the individuals of the colony effect 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- 



