522 ADAM SEDGWICK. 



ceptible of a mechanical explanation. The clean rounded form 

 of the spheres at the moment of division is unlike anything 

 else in the animal kingdom, and is suggestive rather of an 

 intensely active force in the centre of the cell, which compels 

 for the moment the assumption of this form in the protoplasm 

 over which it has dominion, than of a tendency inherited from 

 an adult ancestor. I would refer in this connection to Brook's 

 observations on the total segmentation of Lucifer (No. 5). 

 He describes how, at the moment of activity, the segments 

 round themselves off, touching only at one point, while in the 

 intervals of rest they flatten out against one another, and 

 possibly become partly fused. The same phenomenon is found 

 in other Crustacea [vide Balfour, No. 1, p. 112), and it 

 seems fairly generally to happen that at the moments of 

 activity the segments round themselves off, and in the intervals 

 of rest flatten out against each other. These facts seem to me 

 to indicate that it may be possible to find a purely mechanical 

 explanation of complete cleavage. However this may be, it 

 seems pretty clear that the holoblastic segmentation of small 

 ova has not the phylogenetic significance usually ascribed to it. 

 To sum up, the ancestral Metazoon has generally been 

 assumed to be a colonial Protozoon, and when we examine the 

 evidence for this view we find that the holoblastic segmenta- 

 tion, which really suggested it, is totally opposed to it ; and 

 further, that the facts of incomplete cleavage which were 

 thought to be opposed to it are somewhat in its favour, though 

 much more suggestive of another view, which I will now 

 consider. 



In Part II of this series I suggested that the ancestral 

 Metazoon was not a colonial Protozoon, but a multinucleated 

 Infusorian-like animal with possibly a mouth leading into a 

 central vacuolated mass of protoplasm, and that evolution of 

 the higher forms has consisted mainly in a definite arrange- 

 ment of the nuclei and of the specialisation of certain of the 

 vacuoles in the internal protoplasm into the cavities of 

 organs, and of the protoplasmic strands between into the walls 

 of the latter, and into nerves, muscles, &c. 



