52 The Unity of the Organism 



interpretation of the inner vesicle so far as Botryllus is 

 concerned, but much ground against it for several other 

 species. Even though the Irishism that the endoderm of the 

 bud is not endoderm but ectoderm, that is, that the inner- 

 derm is really an outer-derm be accepted as true, the real 

 issue so far as this discussion is concerned remains unaf- 

 fected. Whatever the inner layer should be considered as 

 judged by its origin, judged by its developmental potency 

 its endodermal nature is beyond question, for nothing is 

 more certain than that it gives rise to the main part of the 

 alimentary system as, in accordance with the general rule, 

 it ought to. The kernel of the matter is that here is a case 

 in which both the digestive organ and the ner\^ous system 

 arise from the same germ-layer, which is contrary to the 

 almost universal rule. What that layer should be called 

 matters not, as we are now looking at the situation. 



We can see, as intimated at the outset, the probable im- 

 mediate cause of this fundamental modification of the on- 

 togeny. Hjort was the first to dwell adequately on this 

 aspect of the subject. But since my own conclusions were 

 drawn before his memoir reached me and so were wholly in- 

 dependent of his, it will be permissible to present the ex- 

 planation in my own way. This I will do in the original 

 language slightly modified. The ectoderm of the ascidian 

 bud, even at its very beginning, is part and parcel of the 

 ectoderm of the parent, particularly in Goodsiria and Bot- 

 ryllus where, in the absence of a stolon, the budding region 

 is enveloped in the cellulose tunic characteristic of ail 

 tunicata. This is equivalent to saying that the ectodeiTn 

 of the bud is not, even at the very outset, an embryonic 

 structure at all. It is, on the contrary, a differentiated 

 organ whose function is, as in the parent, to secrete the 

 cellulose matrix of the outer tunic. In the performance of 

 this function, it would appear to be vigorously and con- 

 sistently active, for the matrix is large in quantity and prob- 



