18 BULLETIN OF THE 
of the origins of the muscles away from the neck, to suppose that the 
eircumcervical region is derived in that way; for (1) the origins of the 
pyramidal muscles actively migrate away from the neck to a certain 
extent, and (2) the normal growth of the body wall is sufficient to ac- 
count for the carrying backward of the origin of the retractors. 
From the facts already gained it seems clear that the ectocyst (cuticula) 
is first formed at the tip, and then, to meet the wants of the growing 
colony, this is replaced later by a cuticula of different chemical compo- 
sition, which becomes thicker as the body wall grows older. At a late 
stage we find a separation of the thick cuticula itself into two layers, of 
which the outer one is much the more highly refractive. (Plate II. 
Fig. 13; Plate III. Figs. 26, 29.) 
6. DEVELOPMENT or THE POLYPIDE 
We have already (pages 8, 9, Figs. 5, 14, 37) seen how the foundations 
of the polypide are laid by the ingression of cells of the outer layer of 
the body wall pushing before them the mesoderm, and how, finally, those 
cells arrange themselves in a boat-shaped mass to form the inner layer 
of the bud (Plate III. Fig. 21), which possesses no actual cavity, and 
is constantly separated from the external world by the ectoderm which 
remains behind to form the neck of the polypide. Even when a cavity 
is formed later, it does not communicate with the exterior until the 
permanent atrial opening has arisen. The earliest differentiation in the 
bud is, as mentioned by Allmann (’56, p. 36), the formation of a cavity 
which is to become that of the atrium. This cavity is first formed 
at an early stage as an extremely slight fissure in the midst of the inner 
layer. Figure 22 shows a longitudinal section of this stage. Cell di- 
vision is taking place throughout the whole mass, but especially at the 
neck of the polypide, cev. pyd. The position of the cavity is represented 
by the central non-nucleated space, and this gives rise, as the later his- 
tory of development shows, to the atrium and the pharynx. 
Figure 23 represents a stage which is doubtless of short duration, for 
I have found it only twice. The bud is much more developed at the 
1 Such a two-layered condition of the cuticula was long ago described by Reichert 
(70, pp. 265, 266) for Zoöbotryon. He distinguished “ eine äussere, festere, stärker 
lichtbrechende und sprödere Schicht und die innere weichere.” Realizing that the 
“ectocyst” or cuticula undergoes many changes in form, — formation of lateral 
buds, of septe or communication plates, and increase in size of the stolon, — he 
suggested, without having observed the process, that probably during these 
changes the more rigid outer layer disappeared and was replaced by the inner 
softer one. 
