EMBRYOLOGY. 245 



in contact with the epithehal layer in the posterior end of the body (Plate 13, 

 fig. 9), and the only tenable theory that suggests itself is that the terminal cells 

 of the ganglionic cords, those adjacent to the epithelium, have originated from 

 the elements at first surrounded by the terminal ring. 



While the cells of the epithelium with which the ganglionic cords come 

 in contact form a fairly distinct group, they are not depressed and otherwise 

 give no indication of constituting a special sense organ, although I am inclined 

 to look upon them as the future dorso-terminal sense organ characteristic of 

 many adult neomenians. 



The Foot: — The oblique direction of the sections through the oldest embryo 

 renders it practically impossible to determine the exact arrangement of the cells 

 of the ventral surface. Immediately beneath the ventral rod of ganglion cells, 

 and therefore in the mid fine, the cells are clearly defined columnar elements 

 placed approximately at right angles to the surface of the body. A short dis- 

 tance removed on each side the cells of this character become replaced by others 

 of more slender appearance that are incUned toward the posterior end of the 

 body. It thus appears that along the mid-ventral line there is a strip of cells 

 about one sixth of the body diameter in width, that probably becomes the 

 future ventral groove and included fold although there are no indications that 

 these structures exist as yet. At the anterior border of this strip, and con- 

 sequently immediately posterior to the mouth opening, is the anterior pedal 

 gland, consisting of several cells bordering upon a slight depression (Plate 14, 

 fig. 4) — the future pit-like outlet prominent in the adult. Five or six cells 

 contain small quantities of a moderately staining secretion, and are relatively 

 conspicuous objects. 



Shell (?) : As just noted the cells bordering upon the mid-ventral line are 

 inclined posteriorly with reference to the surface of the body, and this appears 

 to be generally true of the whole trunk region. Especially along the mid-dorsal 

 line they are comparatively slender, columnar elements (Plate 14, figs. 3, 4), con- 

 taining distinct spherical or ellipsoidal nuclei and one or two yolk granules 

 each. In the oldest larva these have separated at fairly regular intervals, and 

 originally I was inclined to consider the spaces thus formed as the seat of cal- 

 careous products. There is, however, a lack of any definite cuticular sheath, 

 and no perforation or elevation of the membrane bounding the body, and accord- 

 ingly I am now of the opinion that these spaces are due to methods of prepara- 

 tion of the material. In the posterior end of the embryo there is a distinct slit 

 (Plate 13, fig. 9), extending through three sections or about one fifth the diameter 



