laid down in the clearest way what is now accepted by everyone—­that the presence of walls is of minor importance, and that it is the slimy contents of the cells, what is called “protoplasm,” that is the important element.  He declared that the protoplasm of animals was identical with the protoplasm of plants, and that plants were “animals confined in wooden cases.”  He agreed with Schwann that the cell, using the term to imply the contents rather than the wall, was of fundamental importance, and was the unit of structure of the whole world of life.  On the other hand, he declared that it could not be looked at as the unit of function:  he denied that the powers and properties of a living body were simply the sum of the powers and properties of the single cells.  In this opinion he was not followed by physiologists until quite recently.  For many years physiologists held that cells were units of function just as much as they are units of structure; but in the last ten years there has been a strong return to the opinion of Huxley.

In 1851 two very important memoirs were published in the Transactions of the Royal Society, which contained the results of Huxley’s observations of the interesting animals known as “tunicates.”  The first of these papers begins as follows: 

“The Salpae, those strange gelatinous animals, through masses of which the voyager in the great ocean sometimes sails day after day, have been the subject of a great controversy since the time of the publication of the celebrated work of Chamisso, De Animalibus Quibusdam e Classe Vermium Linnaeana.  In this work there were set forth, for the first time, the singular phenomena presented by the reproductive processes of these animals,—­phenomena so strange, and so utterly unlike anything then known to occur in the whole province of zooelogy, that Chamisso’s admirably clear and truthful account was received with almost as much distrust as if he had announced the existence of a veritable Peter Schlemihl.”

According to Chamisso, salps appeared in two forms:  solitary forms, and forms in which a number of salps are united into a long chain.  Each salp of the aggregate form contains within it an embryo receiving nutrition from the mother by a connection similar to the placenta by which the embryo of a mammal receives nourishment from the blood of the mother.  These embryos grow up into the solitary form, and the solitary form gives rise to a long chain of the aggregate form which developes in the interior of the body.  Chamisso compared this progress to the development of insects.  “Supposing,” he said, “caterpillars did not bodily change into butterflies, but by a process of sexual breeding produced young which grew into the ordinary adults, and that these adults, as indeed they do, gave rise to caterpillars by sexual reproduction, then there would be a true alternation of generations.”  The first generation would give rise to a second generation totally