MUSEUM OF COMrABATIVE ZOOLOGY. 27 



7. Origin of the Muscles. 



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 veiy gradually of less diameter, and 

 ti'ansverse sections at that end should exhibit the ingression (potential 

 invagination) of cells which wei-e 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. x>yd'), 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 III. 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 tlie 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, 9, cev. 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 



