7O MORGAN. [Vou. IX. 
digestive portion by a constriction of the walls in front of the 
mouth. The proboscis vesicle is described as formed from 
mesenchyme cells at the base of the proboscis. The collar- 
cavities are said to arise by a pair of lateral evaginations from 
the archenteron. For the present I accept this account of 
the origin of the second pair only tentatively. The evidence 
furnished by Bateson does not seem to me conclusive for 
accepting his statement. I think the phenomena could be 
explained by a process of delamination or migration, and a sub- 
sequent opening (or perhaps the small openings are artefacts). 
I venture this only as a suggestion, and it is my intention to 
test Bateson’s conclusions by a renewed study of the early 
stages of development of Balanoglossus Kowalevskii. The third 
pair of body-cavities, according to Bateson, arise as evaginations 
of the hind portion of the archenteron. To summarize: A// of 
the five body-cavities of the embryo of B. Kowalevsku arise as 
enterocoels. 
Agassiz did not study the development of the body-cavities 
in the New England Tornaria. In my own paper, written in 
1891, a great deal of attention was paid to the method of 
formation of the body-cavities. The origin of the proboscis 
body-cavity was not seen; and, indeed, no one has as yet 
clearly made out the origin of this pair of cavities. Gdtte’s 
account I believe to be due to a misinterpretation, and Bourne’s 
statement that it is formed from scattered mesenchyme cells 
I cannot accept as proven. On a priori grounds, we might 
accept equally well the evagination of the body-cavity described 
by Gétte, or the mesenchyme origin of Bourne ; the evidence 
for either has not been published. The proboscis vesicle arises, 
as I believe, from mesenchyme cells, although Spengel has 
assigned an ectodermal origin to the organ. The collar body- 
cavities arise by a proliferation of endodermal cells on each 
side of the stomach. Whether these cells are simply pulled out 
of the wall, or whether a delamination takes place, I could not 
fully decide. The last pair of body-cavities arise as solid 
evaginations of the last division (endodermal) of the digestive 
tract. 
