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 w'ith 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 is the trunk amniotic cavity. This 

 corresponds to the atriiim of Bryozoa, and, like it, is lined by ectoderm. 



The concrescence of the lips of the blastopore takes place slightly 

 diflferently in Eudoprocta and Ectoprocta, so that in one case the blasto- 

 pore persists in the region of the permanent mouth, and the procto- 

 deeum appears later ; whereas in the other case the blastopore persists 

 in the region of the permanent anus, and the stomadteum 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 suhoesophageal 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 witli 

 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 YI. Fig. 58, di. 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 



