DEVELOPMENT OF THE CAPE SPECIES OF PERIPATUS. 521 



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 

 (Marshall, No. 21 ; Sollas, No. 34), in Alcyonarians (Kowa- 

 levsky and Marion, No. 17) and most Arthropoda that we 

 shall have to seek the nearest embryological counterpart of the 

 process by which the Metazoa arose from the Protozoa. 

 If this is so, how are we to account for the frequency of the 

 cases in which the furrows, dividing the ovum, completely 

 separate the segments from each other ? In the first place, I 

 would ask, are the cases so numerous as is supposed ? It seems 

 to me extremely probable that it will be found, on renewed 

 investigation, that incomplete cleavage takes place in many 

 forms in which it has been assumed that complete cleavage is 

 the rule. The complete cleavage of small ova is such a striking 

 phenomenon, and so readily lends itself to speculative sugges- 

 tions, and has in this form so dominated the views of mor- 

 phologists {vide especially Flemming's remarks above referred 

 to), that I cannot help feeling that it may, in some cases in 

 which perhaps the observation was difficult, have been assumed 

 to occur on insufficient evidence. And this feeling is rather 

 confirmed by the well-known prevalence of the habit of as- 

 suming cell boundaries when they cannot be seen. Almost 

 every embryological memoir bears on its plates numerous 

 examples of this habit. 



In the second place, it seems possible that the complete 

 cleavage, found so conspicuously in small ova, may be sus- 



