f the structure of the genito-urinary organs of the
frog. Compare these organs of the frog with those of the dog-fish and
of the rabbit. Distinguish in each case the conditions of the two
sexes, and describe briefly the microscopic structure and
development of the ova and of the spermatozoa.

17. Describe, with diagrams, the arrangement of the urinary and
generative organs in the male of (a) the rabbit, (b) the dog-fish, and
(c) the frog; (d) point out the most important differences between
them.

18. (a) Describe the structure of the ovarian egg of the rabbit, (b) and
of the pigeon, (c) and of the frog; (d) from what part of the embryo do
they originate? (e) What is the structure and origin of the ovarian
follicle in the rabbit, and (f) of the ovarian stroma? (g) What is the
"granulosa" and what the "zona pellucida"?

19. Describe the pre-segmentation changes, mode of impregnation,
and early stages of development in the ovum of the frog, as far as the
closure of the neural canal.

20. Illustrate, with diagrams, from the structure of typical organisms,
the principle of repetition of similar parts.

-Note on Making Comparisons_

Students preparing for examinations are frequently troubled by
"comparison" questions. Tabulation is often recommended, but we are
inclined to favour a rather more flexible plan of marking off differences
and resemblances. In tabulation a considerable loss of time is
occasioned by writing down the features of both the things compared,
and this is a serious consideration for the examinee. We advise him
therefore, first, if he possibly can, to draw side by side and in
corresponding positions the two things under consideration, and then,
going over them in a methodical way, to state simply the difference
between each homologous part. We append as examples three test
answers actually submitted (with figures) in "Correspondence" work:--

1. Compare the brain of the frog with that of the rabbit.

In the frog's fore-brain--

The olfactory lobes are fused in the middle line.

There is no corpus callosum, nor is there a middle commissure to
the third ventricle.

The cerebral hemispheres are not convoluted, and, looked at from
the dorsal aspect, do not hide the thalamencephalon and mid-brain.

The pineal gland lies in the cranial wall and not deeply between
the hemispheres, and its stalk is longer and tilts forward.

In the mid-brain--