I am ending this series of articles on Maxwell Savage by discussing one small part of his research on the life of the Common Frog, Rana temporaria, in Britain: frog spawn.
Common Frogs lay spawn in clumps, not as single strands of eggs like the Common Toad, Bufo bufo. Savage realised that because the swollen jelly surrounding the egg is 99% water, the mass of spawn acts as a heat store for the developing embryos within. Unlike water in the pond, the water in the jelly is held and thus constitutes ‘a relatively enormous mass of solidified water’. Therefore, in sunlight the mass of spawn warms up. At night, because the warmed ‘solidified water’ cannot go anywhere, the only way for heat to be lost is by conduction and since water is a very poor conductor the heat is retained. Savage did experiments at home and in the field. He showed that in 63 cases out of 73, the spawn was warmer than the surrounding water. The average difference 0.63°C, not a big difference Savage noted, but one which could have given an important advantage to repopulation after the last Ice Age.
He also investigated the structure of the mass of spawn itself. It is not an amorphous mass but structured in a way ‘which may be likened to a bunch of grapes glued together only where they touch’. Therefore, water circulates freely within the mass of spawn and oxygen for respiration only has to pass across the jelly the distance of one half of the diameter of the individual egg. He demonstrated this structure by dropping Indian ink on a freely floating mass; the ink passed through in a very short time.
The corollary of this demonstration of the structure of the spawn mass is that if a mass of frog spawn is laid or later held in water of insufficient depth the inner channels are blocked by collapse of the mass.
Savage also became involved in studying the events that lead to the ovum being surrounded by coats of jelly on its passage through the oviduct, which swell on contact with pond water after fertilization. I will not dwell on the aspect he examined—the inner gel that surrounds the ovum—but it is of historical interest and possible physiological importance. Savage wrote:
By a coincidence, Dr. Burgess Barnett and I shared a common interest in two unrelated scientific fields—the study of amphibia and of blood-clotting. In May 1939, Dr. Barnett, who had just left the Zoological Society of London, where he had been the Curator of Reptiles, for a similar post at Rangoon, wrote to say that he had ‘‘stumbled upon* a curious property of the frog’s egg. The jelly contained a clotting factor which he likened to the prothrombin of blood. He invited me to collaborate m its investigation, and when I accepted he sent me the only notes that he had. They consisted of some tests on normal and on haemophilic blood. In 1939, the subject of blood-coagulation, although it already had a large literature, had not grown to the enormous extent that it has now assumed, and in the light of more recent knowledge, it is clear that the factor he had discovered was not prothrombin, but a variety of thromboplastin [thrombokinase]. Thromboplastins are rather widely distributed in nature, sometimes occurring in unexpected places, and if this had been the only point, there would have been only a mild interest in the fact. From Dr. Barnett’s letters it seems clear that he had not any idea of the possible function of the factor, and very tentatively he suggested that it might be protective…Before we reached the point of actually collaborating, Dr. Barnett died. There the matter remained, until in planning this chapter I realized the probable function of the factor and the part it may play in the ecology of the frog and very likely in the lives of other species as well.
Savage’s experimental evidence suggested that some form of clotting process was involved in the solidification of the inner layer of jelly. I have found no particularly relevant further research but for several descriptions of various numbers of layers of jelly in different species and some of their molecular components. I will leave the last word on this topic to Savage:
This rather complicated story of the formation of the jelly envelope may, then, be summarized as follows. The vitelli [ova] pass into the oviduct. There they form the nuclei round which collects the insoluble product of a clotting system, in which a factor resembling the thromboplastin of blood contributes to a transformation that has something in common with the similar transformation that occurs when blood clots. The protein is not the same, and the rest of the process probably differs considerably. This protein layer is fairly concentrated. When it has formed, the eggs pass into the ovisac, where they become distributed in a fairly concentrated solution of a different protein. When they are ejected by the frog, the mass passes into water, salt is removed and the protein precipitates, forming the second and outer layer of jelly. Both layers now imbibe water, and in twenty-four hours or so achieve maximum size. In this process, the outer layer is stretched by the swelling of the inner layer, and gives way except at the points of contact of the spheres, where there is no stress. The final result is that a mass of adherent spheres is formed, with channels in between. The final percentage of structural protein in a blood-clot and in frog-jelly is about the same, but the clot and the jelly approach this concentration from opposite directions: the jelly swells, but the blood-clot is formed full size at once and, indeed, tends to shrink.
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This is the last in my series on Maxwell Savage, the forgotten man of herpetology in Britain. After searching for information on Savage and producing a short biography Trevor Beebee wrote:
“Ecology and Life History of the Common Frog” is a seminal achievement, bringing all his earlier work together in one book as a very readable monograph. The research is all the more remarkable because it was carried out in his own time, with no external funding – just his own energy and enthusiasm. Among other things, the book also hints at how dramatically frogs declined in the countryside between the 1930s/1940s, when he found ponds with thousands of spawn clumps, and the late 1950s, when only a few of his old study ponds had any spawn at all.
I place Savage at the pinnacle of the small band of people in Britain who have investigated the lives of amphibians and reptiles. He asked simple, important questions about the life of the Common Frog at all stages of his life history which he attempted to answer by any means necessary. Ronald Henry Maxwell Savage cast light. Every time we see a frog we should remember him, his research, and encourage others to take up the challenge of testing his hypotheses and thereby illuminate still further the life of amphibians and how they work.
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