210 TITTLE RR. [VoL. XII. 
tainly, then, if cases can be found where nature, with her 
conservative and infinitely delicate methods, has entered upon 
the same general line of experimentation, and has carried her 
efforts through to complete success, such cases cannot be too 
carefully studied. 
Let us observe with attention the state of things in the 
Tunicate bud. The ectoderm is part and parcel of the ectoderm 
of the parent (this is strictly true in forms like Goodsiria and 
Botryllus, where no stolon is present ; and is also essentially 
true when the budding is stolonic, since here the ectoderm of 
the stolon is only a prolongation of that of the parent). This 
is equivalent to saying that the ectoderm of the bud is not an 
embryonic structure at all. It is, on the contrary, a differentiated 
organ. Its function in the parent is to secrete the cellulose 
test, and in the bud from the very earliest stage it has the 
same function. The specialization of this secretory function 
must be deep-seated, for, as Hjort has pointed out, the cellu- 
lose character of the test necessitates this. Furthermore, not 
only is the specialization deep-seated, but also there must be a 
great and constant activity of the cells; for not only is the test 
considerable in quantity, but it must be perpetually produced 
through the whole life of the zooids to replace the continual 
waste that is taking place from the external surface. One not 
infrequently finds great quantities of diatomes embedded in the 
surface layers of test, and it is well known that many species 
of animals, particularly small Crustacea, work their way into 
the test of Ascidians and there lead a semi-parasitic life. Even 
where no foreign organisms were present, I have often observed 
the surface test in various species to be eroded and ragged. 
The ectoderm, then, has a well-established physiological 
function to perform in the bud from its very earliest stage of 
development. How is it with the endoderm? It is scarcely 
possible to see how a structure could be more favorably situated 
for retaining, so far as its functional relation to the organism 
as a whole is concerned, an undifferentiated character than is the 
‘endoderm ” in the early bud. Not only is it wholly protected 
from contact with the external world, it being enclosed in the 
ectodermic vesicle, but it has nothing to do in the preparation 
