24 BULLETIN OF THE 
My conception of the relation of the processes of atrium formation and 
gastrulation will be more easily understood if we compare the formation 
of the polypide with the early stages in the development of the egg of 
Sipunculus, as given by Hatschek ('83, pp. 78-81). Here gastrulation 
occurs only in the depths of the invagination at the vegetative pole ; the 
more superficial part of the cavity isthe trunk amniotic cavity. This 
corresponds to the atrium of Bryozoa, and, like it, is lined by ectoderm. 
The concrescence of the lips of the blastopore takes place slightly 
differently in Endoprocta and Ectoprocta, so that in one case the blastd- 
pore persists in the region of the permanent mouth, and the procto- 
dieum appears later; whereas in the other case the blastopore persists 
in the region of the permanent anus, and the stomadeum arises later. 
In all Bryozoa, however, the main ganglion arises in the region of con- 
crescence, precisely as the ventral nerve cord is now known to do in most 
invertebrates. Thus additional support is given to the idea that the 
ganglion of Bryozoa is a subesophageal one. 
4. Histology of the Buds. 
The histological structure of the branches is exactly like that of the 
parent stalk. The minute structure of the stolons deserves a word of 
description. The base from which the individuals arise is filled with 
mesenchymatous cells, which at the distal, growing end are closely 
packed and deeply stainable (Plate VI. Figs. 51, 57, sto.). Some of the 
cells are differentiated into muscle fibres, which run up into the stalks 
of the individuals that have arisen from the stolon (Fig. 57, mu.). The 
epithelium at the distal end of the stolon is composed of cuboidal or 
slightly elongated cells. At the base of the stolon the epithelial cells 
are greatly elongated and closely packed together (Figs. 51, 57, cl. sec.). 
At the point of attachment to the parent stem the stolon is consider- 
ably constricted, the epithelium forming an inward fold surrounding a 
small opening through which the parenchymatous tissue of the bud and 
the mother stalk are continuous (Plate VI. Fig. 58, dé. sep.). Between 
the layers of this circular fold is secreted a cuticular disk, perforated at 
its centre. 
5. Formation of New Stocks. 
The development of Urnatella from the egg has never been seen, and 
I was naturally anxious to get embryological material. On looking over 
my preserved material I found no trace of eggs, although there were 
many ripe males. I have no direct knowledge, therefore, as to whether 
