56 The Unity of the Organism 



the two cases. In the first place, it may be reasonably 

 doubted whether the thin cuticular covering produced by the 

 outer layer of the body wall in the bryozoan involves as great 

 a degree of specialization either in nature of product or in 

 extent of activity as does the far more voluminous "test" 

 material produced by the ascidian ectoderm. In the second 

 place, the so-called mesodermal layer of the bryozoan body- 

 wall surely has no such direct and intimate connection with 

 the parent polypides, i.e. the other members of the colony, 

 as does the "cloison" or inner tube of the ascidian colony 

 from which the inner vesicle of the bud is produced. We may 

 consequently surmise that in the bryozoan as in the ascidian 

 the layer that is most available because of being least fully 

 occupied with activities pertaining to the parent organism 

 is most largely drawn upon in bud propagation. A kind 

 of balance between the functions of growth and maintenance 

 on the one hand, and propagation on the other, is struck in 

 each case although this implicates the germ-layers in op- 

 posite ways in two cases. 



