Text Book of Biology Part 15

And the world is not made and dead like a cardboard model or a child's toy, but a living equilibrium; and every day and every hour, every living thing is being weighed in the balance and found sufficient or wanting.

Our little book is the merest beginning in zoology; we have stated one or two groups of facts and made one or two suggestions. The great things of the science of Darwin, Huxley, Wallace, and Balfour remain mainly untold. In the book of nature there are written, for instance, the triumphs of survival, the tragedy of death and extinction, the tragi-comedy of degradation and inheritance, the gruesome lesson of parasitism, and the political satire of colonial organisms. Zoology is, indeed, a philosophy and a literature to those who can read its symbols. In the contemplation of beauty of form and of mechanical beauty, and in the intellectual delight of tracing and elucidating relations.h.i.+ps and criticising appearances, there is also for many a great reward in zoological study. With an increasing knowledge of the facts of the form of life, there gradually appears to the student the realization of an entire unity shaped out by their countless, and often beautiful, diversity. And at last, in the place of the manifoldness of a fair or a marine store, the student of science perceives the infinite variety of one consistent and comprehensive Being-- a realization to which no other study leads him at present so surely.

To the student who feels inclined to amplify this brief outline of Vertebrate Anatomy, we may mention the following books: Wiedersheim's and Parker's Vertebrates, Huxley's Anatomy of the Vertebrata, Flower's Osteology of the Mammalia, Wallace's Distribution, Nicholson and Lyddeker's Palaeontology (Volume 2), the summaries in Rolleston's Forms of Animal Life (where a bibliography will be found), and Balfour's Embryology. But reading without practical work is a dull and unprofitable method of study.

_Questions on Embryology_

[All these questions were actually set at London University Examinations.]

{In Both Editions.}

1. Describe the changes in the egg-cell which precede fertilization; describe the process of fertilization and the formation of the primary cell-layers, as exhibited, in three of the animal types known to you.

What is the notochord, and how is it developed in the frog?

2. Describe the early stages in the development of the egg of the fowl as far as the closure of the neural groove. How do you account for the primitive streak?

3. Describe the cleavage and the surface appearances of the egg of the frog and of the rabbit, up to the time when the first gill-slits appear in the embryo. Give ill.u.s.trative diagrams of what you describe.

4. Describe the structure and cleavage of the ovum (a) of the frog, (b) of the fowl, and (c) of the rabbit. (d) Explain as far as possible the differences in the cleavage of these three eggs. (e) Point out how the embryo is nourished in each case, and (f) describe the const.i.tution of the placenta in the rabbit.

5. (a) What are the protovertebrae? (b) How does the notochord originate in the frog? (c) How are the vertebrae laid down in the tadpole? (d) Describe the vertebral column of the adult frog. (e) In what important respects do the centra of the vertebrae of the frog, the dog-fish, and the rabbit differ from one another?

6. Give an account of the more important features in the development of the frog.

7. What temporary organs are developed in the embryo frog which are absent from the embryo bird and mammal, and what in the two latter which are absent from the former?

8. Draw diagrams, with the parts named, of the heart and great arteries of the frog, giving descriptions only in so far as is necessary to explain your diagrams; trace the development of these structures in the tadpole; point out particularly in which of the embryonic visceral (branchial) arches the great arteries of the adult run.

9. Trace the history of the post-oral gill-slits and their accompanying cartilaginous bars and vascular arches in the frog, fowl, and rabbit.

10. Give a short account, with ill.u.s.trative figures, of the mode of formation of the primary germinal layers in amphioxus and in the frog.

What explanation can you give of the differences between the two cases?

11. Give a short account, with diagrammatic figures, of the princ.i.p.al changes which occur in the circulatory and respiratory organs during the metamorphosis of the tadpole into the frog.

12. How do protozoa differ from higher animals (metazoa) as regards (a) structure, (b) reproduction? Compare the process of fission in an amoeba with the segmentation of the ovum in amphioxus, pointing out the resemblances and differences between the two cases.

-Miscellaneous Questions_

[Most of these questions were actually set at the Biological Examinations of London University.] {In Both Editions.}

1. Describe (a) the digestive, (b) the circulatory, (c) the excretory, and (d) the reproductive organs of the amphioxus.

2. Describe the stomach and intestines of the dog-fish and rabbit, and point out in what way their differences are connected with diet.

3. Describe the mechanism of respiration in the adult frog, and contrast it with that of the tadpole.

4. Give an account of the structure of the epidermis and its outgrowths in the frog and the rabbit.

5. Describe the organs of circulation (heart and main arteries and veins) and respiration in the frog in its mature and immature states.

6. Give a brief account of the physiology of respiration. Describe fully the means by which respiration is effected in the following animals:-- frog, amphioxus, rabbit, and dog-fish.

7. Describe the minute structure of the blood of the rabbit, frog, and amphioxus.

8. Describe and ill.u.s.trate by means of sketches the chief points of difference between the skeleton of the rabbit as a typical mammal, and that of the common frog as a typical amphibian.

9. (a) Explain what is meant by the term "central nervous system."

(b) Describe the tissue elements which enter into its composition.

(c) Explain, as far as you can, the function of each structure described. (d) How is the central nervous system developed in the frog, and (e) in the rabbit? (f) What conclusions may be drawn from the facts stated as to the origin of the central nervous system in evolution?

10. Give an account of the structure (including histology) and of the functions of the spinal cord and spinal nerves of the frog.

11. Give a description of the minute structure and chemical characters of the following tissues as seen in the frog:-- cartilage, bone, muscle. From which of the primary cell-layers of the embryo are they respectively developed?

12. What substance is excreted by the renal organ of a frog, and what relation does this substance bear to the general life of the organism?

Describe the parts by which similar excretion is believed to be effected in amoeba, hydra, earthworm, mussel, and lobster.

13. Describe, with ill.u.s.trative sketches, the structure of the connective tissue, cartilage, and muscular tissue of a frog. Also describe the structure of the muscular tissue of the lobster and snail.

14. Give in account of the more important features in the development of the frog.

15. Describe and compare the structure of the renal organs in a frog and a rabbit.

16. Give an account of 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 s.e.xes, 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 ca.n.a.l.

20. Ill.u.s.trate, with diagrams, from the structure of typical organisms, the principle of repet.i.tion 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 h.o.m.ologous 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.