N 



50 The Unity of the Organism 



tern of ideas set forth in this treatise. Put into a nutshell, 

 the case is one in which the ontogeny or individual develop- 

 ment being much out of the ordinary, several of the germ- 

 layer relationships prevailing in ordinar'y ontogeny are 

 profoundly modified, the end-results being the same as that 

 resulting from an ordinary development. To be explicit 

 on a single point, an instance is presented in which the 

 nervous s} stem arises not from the ectoderm in accordance 

 with the general rule, but from the endoderm, this profound 

 deviation from the typical being explicable, seemingly, from 

 the generally different entogenetic course followed in blasto- 

 genesis. The case, well known to embryologists but insuf- 

 ficiently heeded, is one of bud propagation in some of the 

 compound ascidians. I was not the first to observe the 

 uniqueness in this form of development ; but since in one of 

 the instances studied by me the facts are probably clearer 

 than in any other that has been examined, it will be best 

 to present in the barest outline only, the evidence furnished 

 by this one case. Full details are in my memoir.^ 



{a) Evidence From Bud Propagation in Compound Ascidians 



We will confine our attention almost entirely to the one 

 species, Goodsiria dura (according to the later classifica- 

 tion Metandrocarpa dura), an abundant species on the 

 coast of California. To understand the particular points 

 with which we are concerned, it Avill be necessary to say a 

 few words about bud formation and development in this 

 grou]^. The buds are not produced by "Stolons" as they 

 are in most bud-propagating ascidians, but each blasto- 

 zooid, as the bud-individuals are called, arises separately 

 and directly from its parent zooid. It forms at the anterior 

 end of the parent and in such a way as to be two-layered 

 from the very first, the layers being ectoderm or outer 

 laver^, and endoderm or inner layer. The bud when first 



