No.1.] BUDDING IN GOODSIRIA AND PEROPHORA. i eel) 
buds of genera no farther removed from this genus than are 
Botryllus and Polyclinum. J agree that the fact has very little 
significance, but would point to a different reason for so regard- 
ing it. The posterior communication is always transient and 
without physiological importance. The hypophyseal duct 
forms from the dorsal wall of the inner vesicle at an early 
stage of development of the bud, before the limits of the peri- 
branchial sacs are sharply established ; and since their limits 
on the dorsum of the vesicle are always very close to the 
posterior extremity of the duct, a slight variation in the relative 
position of the point at which the duct terminates, and the 
course of the partitioning folds of the peribranchial sacs; or of 
the time of closure of the duct opening; or the formation 
of the folds, would determine whether the posterior opening of 
the duct should be into the primitive inner vesicle, the branchial 
sac, or one or the other of the peribranchial sacs, or into the 
atrium. I shall show presently that there is a brief period in 
the history of the duct in Goodszvia when it also has both an 
anterior and a posterior opening. The posterior opening 
closes, however, at a stage so early in the development of the 
peribranchial sacs, that we can only regard it as pertaining to 
the primitive vesicle itself. 
As regards the later history of the duct as such, little more 
need be said. In the nearly adult state its dorsal wall, Fig. 50, 
Pl. XV, is exceedingly thin, it being but one layer of cells 
thick, and the cells of this one layer are considerably flattened, 
and their nuclei are far apart. The ventral wall, on the con- 
trary, remains, at least in the oldest stages of which I have 
made sections, several cells thick. Attention may here be 
called to the entire absence of the hypophyseal gland. 
The formation of the duct as I have here described it differs 
in some unimportant particulars from its formation in Bozryllus, 
where, according to Hjort and Pizon, it is not at its begin- 
ning a groove, but an evagination of about equal diameters 
transversely and longitudinally. This evagination then grows 
out into a forwardly directed pouch, the blind tip of which 
fuses later with the wall of the branchial sac, and its lumen 
gains communication with this sac by a perforation at the 
