rather, with that portion of the animal's exterior
enclosed in by the atrial wall; compare Section 6).

(c) The general epithelial lining of the atrium.

The reproductive organs (Figure 4, Sheet 20, g.) are masses of
cells situated in an isolated part of the coelom in the atrial folds, and,
having no ducts, their contents must escape into the atrium by
rupture of the body-wall. Thence they escape either by gill-slits,
pharynx and mouth, or, more generally, through the atrial pore. The
animals, like all the vertebrata, are dioecious, i.e., male or female.

Section 10. The endostyle (end.), in Figures 3 and 4, is a ciliated
path or groove on the under side of the pharynx, which is generally
supposed to represent the thyroid gland of vertebrates. The vertebrate
thyroid, early in development, is certainly an open and long narrow
groove in the ventral side of the pharynx. The hyper-pharyngeal
groove (h.p.) has been in the past compared to the pituitary body,
but there is little doubt now that this structure is represented by the
ciliated pit.

Section 11. The student is advised to revise this chapter before
proceeding, and to schedule carefully the anatomical features under
the headings of (1.) distinctly vertebrate characters, (2.) characters
contrasting with the normal vertebrate structure, (3.) facts of doubtful
import, with the suggestions given in the text written against them.

2. _The Development of Amphioxus_

Section 12. The development of amphioxus, studied completely, is at
once one of the most alluring and difficult tasks in the way of the
zoologist; but certain of its earlier and most obvious fasts may very
conveniently be taken into consideration now.

Section 13. The phenomena of the extrusion of polar bodies and
fertilization are treated of later, and will, therefore, not be considered
now. We will start our description with an egg-cell, which has
escaped, of course, since there are no genital ducts, by rupture of the
parent, has been fertilized by the male element, and is about to
develop into a young amphioxus. It is simply a single cell, with some
power of amoeboid motion, a single nucleus and nucleolus; and in
amphioxus its protoplasm is clear and transparent. Frequently ova are
loaded with granules of food store (yolk), which enable the young
animal to go far with its development before it is hatched and has to
begin fending for itself. Such an ovum as that of our present type,
howe