ANATOMY AND DEVELOPMENT. 221 



A striking example is afforded by the body-cavity 

 of the worm-like Balanoglossus, of which we shall speak 

 later. 



Here, according to BATESON, the cells lining the cavity 

 are continually budding off daughter-cells, which fall into 

 the cavity, and eventually almost entirely fill it up with 

 ntesenchymatous tissue. In this case, therefore, mesen- 

 chyme and an epithelial wall coexist. 



Similarly, the epithelial sclerotomc of Amphioxus is rep- 

 resented by a mesenchymatous sclerotome in the higher 

 Vertebrates. It is not necessary to multiply instances, 

 but many others could be adduced. 



If, now, this disintegration of parietal and visceral layers 

 of the mesoderm, which we have imagined above to take 

 place in the ontogeny of an animal like Amphioxus, be 

 supposed to be thrown back in the development, or, in 

 other words, abbreviated to such an extent that the pre- 

 liminary formation of a continuous coelomic epithelium no 

 longer takes place, we should have precisely those condi- 

 tions which we actually find in existing Ascidians. 



As in the cases above quoted for purposes of illustra- 

 tion, so in the Ascidians the mesenchymatous condition 

 undoubtedly originated ancestrally from what we may call 

 an epithelial condition. 



In the Ascidians we may conclude, therefore, that while 

 ontogenetically the mesenchymatous condition is to all 

 intents and purposes primary, from a phylogenetic point 

 of view it is pre-eminently secondary or cenogenetic. 



Having made the reservations implied in the above 

 statements, we may confidently assert that as a whole 

 the body-cavity of the Ascidians is homologous with the 

 ccelom of Amphioxus, and we may define the former as 

 a ccelom in which the cells, instead of associating together 



