MUSEUM OF COMPARATIVE ZOOLOGY, 59 
and secondarily acquired connection with it, or that it had a double 
origin. 
To Nitsche (’71, pp. 456-463) belongs the credit of having first described 
the histological changes in the origin and development of the polypide 
of marine Bryozoa, particularly with reference to the part which the 
germ layers play in that process. He says (71, p. 456): “Die Anlage 
des Polypids erscheint zunächst als eine Wucherung der Zellschicht der 
Endocyste in der Mitte der Hinterwand der Knospe, und zwar in dem 
Winkel, den die Hinterwand mit der oberen Wand macht. Bald ordnen 
sich die Bestandtheile des regelloşen Zellhaufens in zwei deutlich geson- 
derte Schichten, und wir sehen nun einen rundlichen Körper, beste- 
hend aus einer äusseren einschichtigen Zellschicht, welche sich scharf 
absetzt gegen die das Innere des Körpers bildenden Zellen.” 
This stood until a year ago as the most satisfactory description of this 
process in the adult stock. The appearance within the last year of the 
two papers of Prouho (°90) and Seeliger (90) marks a distinct epoch in 
the advance of our knowledge concerning the origin of the polypide in 
Gymnolsemata. The paper of Prouho treats of the process in the case of 
the primary polypide of the metamorphosing larva of Flustrella, that of 
Seeliger in the case of the young (practically adult) stock of Bugula. 
According to both authors, the polypide arises from the body wall by an 
invagination of it, and its two layers are from the first distinct and 
separate, and go to form the two layers of the adult polypide, and the 
whole of those two layers. The outer layer of the body wall gives rise 
to the outer layer of the tentacles and the lining of the alimentary tract, 
and the inner layer of the body wall gives rise to the mesodermal lining 
of the polypide. Prouho alone is cognizant. of the method of origin of 
the ganglion, and in addition there are several points of difference be- 
tween these two authors concerning the development of other organs, to 
which I shall refer in the proper place. Thus the latest studies have 
confirmed the assertions of Nitsche, that the polypide arises from a 
single centre of proliferation of the body wall; they have made an ad- 
vance in this, that they have shown that the two layers of the bud do 
not become secondarily differentiated from a single cell mass, but are 
respectively derived from the two cell layers of the body wall. My own 
studies have led me to the same conclusion on this point. 
Figure 75 (Plate IX.) is a vertical radial section through the margin 
of an adult Flustrella stock. The ectoderm is relatively thick at the sole 
(sol.) and margin, and very greatly thickened at the point marked qm. 
Here two layers, sharply separated, are apparent. The cells of the outer 
