48 SEGMENTATION AND FORMATION OF THE LAYERS. 



tissue cells, cartilage cells, epithelial cells, &c. And not only 

 may tlie cells of one tissue be continuous with each other, but 

 they may also be continuous with the cells of other tissues. 

 For instance, I may refer to Fraipont's (2) work on the nervous 

 system of the Archiannelida. He describes an intermuscular 

 nervous plexus which is continuous with the muscle-cells and 

 with the surface epithelial cells (2, PL 13, figs. 11, 16). 



Instances of this kind might be multiplied from recorded 

 observations, and are being multiplied day by day by histolo- 

 gical observers to such an extent, that we are almost, if not 

 quite, justified in regarding the body of an adult animal as a 

 syncytium. It is true that the cells of the blood and lymph, 

 and the ripe generative cells, are completely isolated. But the 

 former, in their first stages of growth, form part of the syncy- 

 tium ; as in all probability do the latter also.^ 



This continuity, which for a priori reasons we should 

 expect, has hitherto been regarded as a fact of little morpholo- 

 gical importance and relegated to the category of secondary 

 features. The ovum, it is said, segments into completely 

 isolated cells; and the connection between these is a secondary 

 feature acquired late in development. It has always been con- 

 sidered that the first stage in the evolution of the Metazoa 

 was a colonial Protozoon, i. e. a mass of perfectly isolated uni- 

 cellular organisms derived by complete division from a single 

 cell. 



Now, while I do not wish to exalt the facts of the cleavage 

 and early development of Peripatus above recorded to a posi- 

 tion of undue importance, or to maintain that of themselves 

 they are sufficient to destroy this conception of the origin and 

 structure of a Metazoon, I think I am justified in pointing 

 out that if they are found to have a general application, our 

 ideas on these subjects and others connected with them will 

 have to undergo a considerable modification. 



The ancestral Metazoon will no longer be looked upon as a 

 colonial Protozoon, but rather as having the nature of a multi- 



1 I may refer in this connection to the processes of the follicular cells which 

 perforate the zona of a mammalian ovum, 



