Stories of the Universe: Animal Life

by B. Lindsay

If we want to know not only the microscopic structure of organs, but also their shape and position in the body, and their relations with other parts, we must have every successive section carefully preserved, and the whole row arranged in correct successive order; the physiologist may often content himself with single sections; the zoologist must have rows and rows of them. What a task this was, a quarter of a century ago, for scientists who cut their sections by hand!

Let us, however, describe first the way in which objects are prepared for section-cutting—whether by hand or by machine. It has already been noticed that animal substances contain a quantity of water, and therefore will not keep. The same circumstance renders them soft and squashy, so that the sharpest razor in the world, in cutting a section, must necessarily do more or less damage to the structure of the delicate tissues. The water is held in the meshes of the tissues just as it is held, for example, in the meshes of a sponge. Now, if we were dealing with the sponge, we could get it to absorb any other fluid substance besides water; we might choose one that would prevent decomposition; we might choose one that would go harder by cooling; so as to change the sponge into a strong solid block that could be knocked about without sustaining any damage. This is exactly what we must do with our animal tissue to prepare it for section-cutting; and the most convenient fluid for the purpose is melted wax. But whereas we might take our sponge out of water, squeeze it dry, and dip it straight into melted wax, we can by no means do so with our animal tissues. For one thing they usually cannot be squeezed, and where they can, they would of course be irretrievably ruined by such a rough process. Even the transference of the specimen from one fluid to another of very different qualities and density, would deface the tissues. Cells would burst, or be squeezed out of shape, and organs would be loosed from their right position by the currents set up in all parts of the specimen, under such circumstances. We must, therefore, try to get rid of the water by degrees. This may be done by gradually adding alcohol, a fluid which may be diluted with water in any proportion. We begin with a comparatively weak solution of alcohol, say about fifty per cent., and immerse the specimen in this for some little time. The time required depends somewhat upon the size of the specimen; if a large one, a new fluid will take longer to filter through it. Then we must change this solution of alcohol for stronger ones, say seventy per cent. and ninety per cent. successively, and finally to absolute alcohol. By this time the alcohol will have removed almost nearly all trace of water from the specimen. The latter is now nearly but not quite ready to be imbedded in melted wax; but first we must soak it for a while in a fluid intermediate in thickness between the alcohol and the wax, and capable of mixing in a friendly manner with both. Then it goes into a bath of melted wax, and is kept for hours at a stated temperature until the wax permeates it thoroughly. Then the melted wax and the specimen along with it is poured into a little mould and left to cool. The block of wax containing the specimen is cut down to a quadrangular shape, and is now ready for section cutting. In old days the block was placed in a stand, and successive sections were cut from it by hand with a razor. But this process is much too slow for modern days. Machines called microtomes (i.e. cutters of small parts) have been invented, and of these there are several kinds—in all, however, the razor is worked by machine and not by hand, so as to secure steadiness and a uniform thinness of the sections. The old microtomes threw off each section separately; but now matters are so arranged that the wax of each section adheres to that of the next, and the whole series of sections forms a continuous ribbon of thin wax. A large specimen, affording a number of sections, thus results in a ribbon of considerable length. Further processes are now required to fit the sections for the microscope. The ribbon must be divided into successive pieces of a length determined by that of the slides to be used. These are mounted in order on the slides, steps are taken to melt away the wax from the sections, the latter are covered with Canada Balsam surmounted by a glass cover slip, and left for some time to dry. After this they are ready for examination, and it is only now that the work really begins. All that has gone before is mere handicraft; it is time now for science to be called into play.

Fig. 47.—Sections of Embryo Chick, eight days old. A slide mounted for microscopic examination, showing sections arranged in ribbons.

The sections must be compared with others of the same kind which have been cut before. Do they entirely resemble these, or is there a difference somewhere? Happy the man who finds that his sections represent a fresh stage, perhaps older or younger than any that has been seen before in the history of the particular animal which is under investigation. Happier still the man who has succeeded in getting hold of an animal which has not been described before. He will make haste to write a full description of it, illustrated by drawings; to found a new theory on it, if that can possibly be done; and to publish it to the world. It will go all over the globe. To every country in Europe; to the centres of learning in the United States; to universities in New Zealand and Australia, and our other colonies; and perhaps even to "Far Japan."

When in his turn he receives publications from all countries, written in all languages, he is in a position to realise the very great advantage (referred to in an earlier page, [p. 31]) that results from the use of the learned tongues, in the terminology of zoological science. For the educated classes in all countries are equally acquainted with these; and when half of a sentence consists of words of Greek or Latin derivation, the labour of translation from a foreign tongue is necessarily greatly lightened. To no writer is this advantage of so great importance as to the Englishman, who is usually less familiar with the tongues of other nations than his colleagues abroad. It will easily be understood that in the world of zoology, there is no "predominance of the English-speaking races." Far from it. German is the language which supplies the fullest literature of every scientific subject; and in England even our text-books are, for the most part, translated from the German. German, in short, is to the seeker after Knowledge, what English is to the seeker after Money.

Let us now pause a moment to consider how large a number of different industries profit by the labour of the zoologist. First there is the shipping trade; for, of course, all specimens from foreign lands are brought by sea. The chemist supplies preservative substances, and reagents used in the preparation of objects for the microscope. The construction of microscopes is a profession in itself, and one which employs many industries; for the making of a microscope includes not only the work of the optician, but also that of the artificer in brass, and of many other handicraftsmen. The glass-worker supplies "slides," that is to say, the thin pieces of glass upon which objects for the microscope are placed, and "cover-slips," the little sheets of thinner glass which are laid over them; and, besides these, the bottles in which specimens are placed. Then comes the microtome, already spoken of, by means of which sections for the microscope are cut; how many skilled workmen have been engaged in the construction of its parts! Sheffield, perhaps, has supplied the razor which it holds, as well as the instruments for the dissection of the larger zoological specimens. We have already spoken of the laboratory servants, and the bone-articulators and skin-stuffers, who are personally and directly employed by the zoologist; and of the artists and photographers who depict his specimens, or perhaps copy his drawings. We must add to the list of the zoologist's helpers, last, but not least, the printer who "sets" the learned treatise in which the final result of his work is usually embodied; and attendant on the work of the printer is that of the bookbinder. With the bookseller the zoologist has but little to do; the general public, even the reading public, has no knowledge whatever of the writings of the zoological specialist. They are addressed to his equals and co-workers, not to critics and reviewers. Their publication is provided for, not by the law of supply and demand, but by the funds of the learned societies and the universities. It is only occasionally that a writer arises who is able and willing, like Huxley or Darwin, to express himself in a book that the general public can read; and it is only after a lifetime of detailed work, such as is understood only by the specialist, that writers like these think it fitting to lay the results of their labour before outsiders.

The librarian, finally, must not be forgotten, in making up our list of the zoologist's helpers. The preservation and cataloguing of zoological literature is obviously a task all the more important, because, as we have already stated, zoological writings are not regulated by the law of supply and demand. A very little paper, read to a very small meeting of a learned society, and wholly ignored by the world at large, may contain facts priceless to the world of science. It is on the accurate and painstaking work of the librarian, who preserves and catalogues small things as conscientiously as large ones, that we rely for the completeness of our record of zoological knowledge. Such work has at all times been carried on in the libraries of our universities; but at the present time there are in existence libraries specially devoted to zoological literature alone.

The museum, again, must not be forgotten, in which our man of learning stores his specimens, duly labelled and arranged. Here, again, is a staff of curators and sub-curators; and, under their direction, work for various workmen, and for perhaps even a humble charwoman to dust the shelves.