208 RITTER. (Vou. XII. 
indicated. In Pevophora, on the other hand, the ganglion is 
not fully separted from the duct till the peribranchial sacs are 
well developed, and the stigmata have begun to form. Like- 
wise the endostyle is well advanced to its final form. 
The first differentiation that occurs in the solid, irregular 
cell mass, that at first constitutes the Avz/age, consists in the 
modeling of this into a quite regular cylinder, in which there 
soon appears alumen. The wall of the tube thus formed is 
several cells thick, and is of about equal thickness on all sides 
throughout its length. Before the lumen is wholly completed 
a fusion between the walls of the anterior end of the tube and 
of the branchial sac takes place, the fused area becomes per- 
forated, and thus the hypophysis mouth is produced. The 
formation of the ganglion begins by a rapid proliferation of 
cells in the dorsal wall of the duct. Nearly the entire length 
of the duct participates in the process, although in the nearly 
adult condition the ganglion does not extend entirely to either 
end of the duct. Fig. 76 represents a transverse section of a 
duct at an early stage in the development of the ganglion, and 
Fig. 75 a longitudinal section of a much later stage —a stage, 
in fact, when the ganglion is almost completely separated from 
the duct. 
The intervening stages I have not thought it necessary to 
figure. They occur in many of my sections, however, and the 
whole process is very clear, and easily observed. The separa- 
tion takes place considerably earlier in some buds than in 
others, and in a majority of cases it is completed before a 
stage so late as that shown by Fig. 75. 
I may, perhaps, here refer again to the fact that the ganglion 
in this species develops on the dorsal side of the duct, while it 
develops on its ventral side in Goodszria. 
It is unnecessary to follow the development further. In 
a short time the ganglion reaches a diameter considerably 
exceeding that of the duct, and it acquires the characteristic 
mantle of ganglion cells. 
A single developmental point has been omitted that ought, 
perhaps, to be mentioned. In a few individuals I have noticed 
a distinct thickening in the ventral wall of the tube. Where 
