MUSEUM OF COMPARATIVE ZOOLOGY. 97 
best figured series in which to trace the homology of poles is that shown 
by Repiachoff ('80, Taf IIT.) for Bowerbankia. So far as the figures 
go, one would conclude that Figure 10 A and its predecessors were 
oriented in the opposite direction to Figure 11 and its successors, 
which would result in placing the pole of ingression (Fig. 9) at the 
aboral pole of the larva, — the pole which here, as in all other Gymno- 
lemata, and, I believe, in Phylactolemata also, gives rise to the primary 
polypide. I have given above additional evidence for this conclusion, 
in my argument to prove the homology of the larvee and larval organs in 
Phylactolemata and Gymnolemata. 
The polypides arise in Phylactolemata at the pole of ingression, which 
is probably homologous with the aboral pole of Gymnolemata. The pole 
of ingression, or the region of the lips of the blastopore, must be regarded 
as being a region of less pronounced differentiation than the rest of the 
Its cells cannot be said to be either ectodermal or ento- 
gastrula. 
It is an interesting fact, that it is just these indifferent cells 
dermal. 
— not yet either ectoderm or entoderm — that give rise to the inner 
layer of the polypide, from which organs usually considered ectodermal 
as well as those considered entodermal arise. 
My conclusion, then, the objections to which I fully realize, may be 
stated in the following words: The inner layer of the polypide bud is 
composed of cells derived from the rim of the blastopore. Such cells are to be 
regarded as still indifferent, and as first becoming differentiated into ecto- 
derm and entoderm in the formation of the young polypide. 
Just when and where, on this hypothesis, the differentiation into 
ectoderm and entoderm occurs, is an important question ; but unfor- 
tunately I cannot answer it decisively. Tt may be pointed out, however, 
that it has now been shown for most Ectoprocta that the lining of the 
middle part of the alimentary tract is formed independently of the 
œsophagus, and by an actual or potential outpocketing of the primitive 
simple sac of the bud. In Endoprocta there is a similar outpocketing, 
which, however, arises in connection with the esophagus, and is formed 
independently of the rectum. 
This is perbaps the proper place to ‚all attention to the fact that the 
mesodermal outer layer of the bud has a very embryonic character at 
the budding region. This is indicated by the fact, that in Phylactolemata 
(in which group alone I have studied the subject) eges always arise 
from that part of the cœlomic epithelium which lies in the budding 
region (cf, Plate XI. Fig. 93). In Pyrosoma, also, according to the 
researches of Seeliger (789, pp. 598-602) the mesoderm of the budding 
VOL. XXII — NO. 1. 
