. Chemical embryology. Embryology. 686 THE RESPIRATION AND [PT. Ill also necessary to remember that the unabsorbed yolk-mass will in the early stages be included in the weight estimations, though it cannot be counted as thermogenetic tissue. It is easy to understand the decline in metabolic rate with advancing age, for the surface, the means of exit from and entrance to the body, does not grow as fast as the weight, but it is difficult to understand, on the view held by some physiologists that thermolysis is the cause of thermogenesis, how the embryo can have an increasing metabolic rate
. Chemical embryology. Embryology. 686 THE RESPIRATION AND [PT. Ill also necessary to remember that the unabsorbed yolk-mass will in the early stages be included in the weight estimations, though it cannot be counted as thermogenetic tissue. It is easy to understand the decline in metabolic rate with advancing age, for the surface, the means of exit from and entrance to the body, does not grow as fast as the weight, but it is difficult to understand, on the view held by some physiologists that thermolysis is the cause of thermogenesis, how the embryo can have an increasing metabolic rate in the early stages—as it assuredly does—when every moment the surface/volume ratio is diminishing. The factors controlling the production of heat must be sought somewhere within the body rather than at the surface. Gayda also discussed the interesting relations that exist between the heat-production and the time required to double the weight. Curves for these values are shown in Fig. 135. The S-shaped t nature of the curve is striking, ^ but perhaps the lowering at the f older stages is brought about by I metamorphosis, and so lies out | of the strict part of this discus- .| sion. During the greater part of ^ the larval period, both before ^ and after feeding has com- 5 menced, however, the parallel- J ism between the two curves is close. Obviously, the less heat that is evolved during the doubling of the weight, the more efficient will be the embryo or larva, and the more economically the turnover will be progressing. Fig. 135 shows that the least heat is evolved in the earliest stages, shortly after hatching, so it must be concluded that the greatest efficiency exists then. The most inefficient point would appear to be at the weight of 40 mgm. Whether it is significant that just at this point feeding begins is not clear. These relations are the direct opposite of what has been found to hold in the case of the chick, which at the 3rd day of incubation appears to be v
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Keywords: ., bookcentury1900, bookpublishernewyorkthem, booksubjectembryology
