MUSEUM OF COMPARATIVE ZOOLOGY, 27 
7. ORIGIN or THE MUSOLES. 
a. Retractor. — After its first formation the bud becomes elongated 
in the direction of the axis of the branch. The derivation of this elon- 
gated stage from the much shorter earlier one might be effected in one 
of two ways: either, first, by the ingression of cells from the ectoderm 
at points successively more and more remote from the point of primary 
invagination, the additions to the length of the bud being made by a 
continuation backwards of that process by which the first foundations 
were laid; or secondly, by cell proliferation at the point of first invagi- 
nation pushing the oral end of the buds farther and farther from the 
neck of the polypide. 
I think there can be little doubt that the second is the method 
by which the bud becomes elongated; and for the following reasons, 
(1) The oral end of the bud, on the supposition of continued invagina- 
tion of the body wall, should become very gradually of less diameter, and 
transverse sections at that end should exhibit the ingression (potential 
invagination) of cells which were observed in the earliest stage; but 
as a matter of fact the oral end is abrupt (Plate III. Fig. 22, 23, Or.), 
and no stages of ingression are to be found there. (2) On the first 
assumption, the inner layer of the bud should be at all points in equally 
close relation to the ectoderm of the body wall; on the second, the 
inner layer should be closely connected with the ectoderm at the neck 
of the polypide (Plate III. Fig. 22, cev. pyd.), but elsewhere it should 
be sharply separated from it. As a matter of fact, a sharp line can 
be distinguished, in a sagittal section, separating the inner layer of the 
bud from the overlying ectoderm at all points except at the neck (anal 
part) of the polypide (Plate IIT. Figs. 22-25). Moreover, cross sections 
of the anal part of the bud show the inner layer passing directly into 
the ectoderm, and oralward the outer layer of the bud tends to pene- 
trate more and more between the ectoderm and the inner layer. 
Therefore I conclude that the inner layer of the bud is constantly 
augmented by cell proliferation in its mass, and especially at the neck 
of the polypide, and this explanation also accounts for the active cell 
proliferation observed at the neck in Plate III. Figure 22, cev. pyd. 
Since the polypide later becomes attached to the body wall by the 
comparatively narrow “neck” only (Figs. 7, 9scev. pyd.), a Separation of 
the oral part from the body wall has to take place. This process begins 
at the oral end. In its earliest stages it is indicated by the sharp sep- 
aration of the inner bud-layer from the overlying ectoderm, and the 
