DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 781 
b. Histology. 
By macerating the tissues in Herrwias’ mixture of osmic and acetic acids (see p. 728) 
the entodermic elements can be teased apart, and the muscle-cells completely isolated. 
We find thus that each muscle-fibre is developed from the base of an entoderm cell, 
the fibre and cell together constituting an “ epithelio-muscular” cell or—to adopt the 
more convenient term proposed by Ctaus—a myoblast. 
The myoblasts are of exceedingly diverse forms, as illustrated by the series of 
figures (170* to 170). There is great variation in the length of the fibre, depending 
apparently on the age of the fibre, since the shorter ones are often no thicker than 
the longer. 
The fibres taper towards both ends and are sometimes thrown into transverse folds. 
They consist of a homogeneous highly refractive substance which differs entirely from 
the body of the cell. The latter is composed of a granular substance, and contains 
a distinct rounded nucleus. As to the form of the cell, every gradation may be 
observed between a regular columnar cell planted on the fibre (fig. 170’) and a 
slight accumulation of protoplasm surrounding a nucleus, which is closely applied to 
the side of the fibre (fig. 1707). In all of the forms a delicate layer of granular 
protoplasm often extends for a considerable distance along the fibre (see figs. 
Br Os). 
From these appearances I conclude that the body of the cell always extends at 
first to the surface of the entoderm, the myoblast being at this stage a typical 
“ epithelio-muscular” cell. As the fibre increases in size the body of the cell sinks 
into the entoderm and diminishes in size, the myoblast then becoming an “ intra- 
epithelial” muscular cell (HERtTWIGs). Finally, the myoblast is wholly buried in the 
entoderm, the cell-body dwindles away and a “sub-epithelial” cell results—ze., an 
ordinary nucleated muscle-fibre. This course of development is, however, only 
inferred from the perfect series of forms shown among the myoblasts, since the out- 
lines of the cells cannot be distinguished in the sections with sufficient clearness to 
follow their development. 
The fibres of both layers are at first arranged in simple flat sheets. Later, the 
lamella is thrown into folds, so that both systems of muscles assume a more or less 
arborescent form in sections. The foldings take place in a very peculiar manner, such 
that those of the longitudinal layer alone are visible in transverse sections, and, 
mutatis mutandis, those of the circular layer in longitudinal sections. I have not 
followed in detail the development of these folds, since it has been very thoroughly 
studied by the Hertwic Brothers in the Actiniz. 
Review. 
Both systems of muscles, circular and longitudinal, are formed in the entoderm 
alone, and an ectodermic musculature is entirely wanting, with the possible exception 
of some of the muscles of the tentacles. No other case of a purely entodermic muscu- 
MDCCCLX XXIII, 5 
