The Animal Organhm and its Germ-Layers 53 



ably constantly renewed. This production may, as Hjort 

 had well contended, be compared with the production of 

 horn, or still better, of cartilage matrix by the cells appro- 

 priate to these substances. So the ectoderm has a well- 

 established physiological role to play from the very earliest 

 stage in the career of the bud. Quite otherwise is it with 

 the endoderm. It is difficult to see how a structure could be 

 more favorably circumstanced for retaining, so far as its 

 physiological relation to the organism as a whole is con- 

 cerned, an undifferentiated state than is the case witli this 

 one. It is wliolU^ protected from contact with the external 

 world by being enclosed in the ectodermic vesicle ; further- 

 more, it has little or nothing to do with the preparation of 

 its own nutriment, since it is constantly and completely 

 bathed in the maternal blood. So why should not the pro- 

 duction of structures which in embryogenesis belong to the 

 ectoderm, be here transferred to the endoderm.? And so 

 it is. 



This conclusion is the more justified when one considers 

 how differently circumstanced are the two layers in the 

 embryo. Here the incipient nervous system arises from 

 the ectoderm while the layer is in a strictly embryonic stage 

 and before the endodenn has freed itself from the rich store 

 of yolk material which is passed on to it from the Qgg. We 

 seem to have here an instance in nature where the later 

 functional requirements of the organism as such have run 

 counter to the way in which, through the operation of re- 

 moter hereditary influences alone, development would pro- 

 ceed; and the former have proved more powerful. 



(&) Evidence From Bud Propagation in Bryozoa 



Defiance of the germ-layer doctrine is by no means re- 

 stricted to the gemmiparous ascidians. In bud propagation 

 in bryozoa, a widely different group, departure from the 



