Page:Popular Science Monthly Volume 55.djvu/421

Rh the Ascidians were brought into line as ancestral vertebrates, it was no wonder to find Professor Brooks laboring upon these interesting creatures, but his work in this group started from a different point of view. As early as 1875, when studying in the laboratory of Alexander Agassiz, he contributed to the Boston Society of Natural History a description involving a most novel interpretation of the embryology of a remarkable Ascidian, Salpa. This form is known to many not naturalists as that beautiful animal chain which is sometimes so common in the clear waters of Newport Harbor as to be dipped up in every bucket of water, but more often not there at all. The female buds forth male branches and gives each an egg (which is fertilized to form a second generation of females). There is thus no alteration of sexual and non-sexual generations at all; and, with characteristic appreciation of a paradox, Professor Brooks subsequently emphasized the fact that the poet naturalist Chamisso, in discovering, in 1814, "Alternation of Generations" in Salpa, had discovered a phenomenon where it did not exist, though subsequently found common enough in many other animals. With the continuity of interest so marked in him, the life history of Salpa, as thus revealed, continued to be one of the living thoughts in Professor Brooks's mind for a long period of years, and, with the accumulation of material and results of researches afforded by his summer work, culminated in the monograph Salpa—a quarto of nearly four hundred pages and fifty odd plates—published in 1893, or after nearly twenty years of sustained interest in this complex problem. In this volume we find first a coherent view of the intricate life history of this animal illuminated by such metaphors as make the necessary technicalities both readable and thinkable. For instance, "A chain of Salpa may be compared to two chains of cars on two parallel tracks, placed so that the middle of each car on one track is opposite the ends of two cars of the other track, and each joined by two couplings to the car in front of it on its own track, and in the same way to the one behind it, and also to those diagonally in front of it and behind it on the other track." Again, in speaking of that startling process of egg development that makes the embryology of Salpa one of the apparently insoluble problems of this branch of inquiry, he says: "Stated in a word, the most remarkable peculiarity of the Salpa embryology is this: It is blocked out in follicular cells, which form layers and undergo foldings and other changes which result in an outline or model of all the general features in the organization of the embryo. While these processes are going on the development of the blastomeres is retarded, so that they are carried into their final position in the embryo while still in a rudimentary condition. Finally, when they reach the places they are to