Page:Popular Science Monthly Volume 27.djvu/686

666 Cambridge school—Mr. William Bateson, of Pembroke College—has been twice across the Atlantic, in 1883 and 1884, to the coast of Maryland, U. S. A., in order to study the growth from the egg of Balanoglossus, the most important and (to the zoölogist) entertaining of all worms, since it has gill-slits like a fish and rudiments of a backbone. Mr. Bateson has made and already published (in a special supplement of the "Quarterly Journal of Microscopical Science," 1885) a complete study of the development of this worm. It is perhaps as well briefly to mention here that a "complete study" in these questions means the preparation and preservation in alcohol of hundreds of specimens of different stages of growth (often very minute) of the animal under investigation, and the subsequent cutting into series of consecutive slices, each about an inch thick, of a sample of each of these stages; the scrutiny of these sections with the microscope, and the reconstruction or building up of the actual structure of the animal at each stage by a mental combination of the sections.

The expedition undertaken by Mr. Caldwell (who was aided in his equipment by funds from the Government Grant Committee of the Royal Society) is perhaps the most interesting, because the animals which he has gone to study are of large size and already more or less familial. The Ornithorhynchus and the Echidna are hairy quadrupeds (mammals) peculiar to Australasia, which differ from all other hairy quadrupeds in having, like birds, but a single aperture to the exterior for the intestine and the urino-genital canals, and in having the skeleton of the shoulder-girdle and some other features of structure similar to those of reptiles. Like those of reptiles, their bodies are comparatively cold, instead of being kept to a definite "blood-heat" (100° Fahr.) as are those of all other mammals. It had often been reported, and some kind of evidence had been given to support the statement, that these strange beasts lay their eggs like birds and reptiles, instead of retaining the egg-like structure within the body and allowing it there to develop to a certain condition of maturity as do all other hairy quadrupeds. One of Mr. Caldwell's objects was definitely to ascertain whether these animals lay eggs or not, and, of more importance than that, to examine minutely the whole history of the growth in the egg, and to compare it on the one hand with the corresponding development of birds and reptiles, on the other with that of ordinary hairy quadrupeds or mammals.

Mr. Caldwell has found out all about the eggs of these animals and collected them in quantities. The Echidna lays a single egg y which she then carries about with her in a pouch formed by a fold of skin on the ventral surface of the body, similar to the kangaroo's pouch.

The duck-mole, on the other hand, lays two eggs at a time and does not carry them about, but deposits them in her nest, an underground burrow like that of the mole. Naturalists are awaiting with