SECT. 6] OF THE EMBRYO 927 



interest. It must be stated, however, that the rhythms of perme- 

 abiUty and susceptibility during the cleavage of echinoderm eggs are 

 not regarded as significant by some investigators, who ascribe them 

 to purely mechanical causes associated with the changing shape of 

 the embryo during the cleavage stages. An account of them will be 

 found in the Section on resistance and susceptibility. 



How does the absorption intensity of carbohydrate vary during the 

 development of the chick? The study of the carbohydrate meta- 

 bolism of the hen's egg which I made in 1927 provided the data 

 which were requisite for the answering of this question. Exactly the 

 same procedure which had been previously applied to protein and 

 fat was applied to carbohydrate; the calculations appear in Table 113, 

 and the resulting curve in Fig. 252 alongside the curves for protein 

 and for fat. The difficulty here was to assess the amount of glucose 

 combusted during each period. This can evidently be no more than 

 a rough approximation, for no accurate data exist on carbohydrate 

 combustion, nor is it easy to see how they could be obtained, since 

 sugar is burned completely away to carbon dioxide and water, 

 leaving no end products whose concentration can be measured. 

 Fiske & Boyden pointed out that the combustion of 100 mgm. of 

 glucose would produce 75 c.c. of carbon dioxide, whereas in the first 

 five days the embryo only produces 10 c.c. and Col. 3 was constructed 

 bearing this in mind. All the carbohydrate lost cannot have been 

 combusted. Col. 4 gives the sum of Cols. 2 and 3, i.e. the amount 

 absorbed in each period, and Col. 5 shows the amount remaining 

 outside the embryo. In Col. 6, Col. 4 is expressed in percentage of 

 Col. 5; in other words, this gives the amount absorbed each day 

 in percentage of the amount remaining to be absorbed. Cols. 7 and 8 

 give the weights of the embryo, dry and wet, calculated irom Mur- 

 ray's data and some of mine. Cols. 9 and 10 show the intensity of 

 absorption of carbohydrate calculated for wet and dry weight. 



The two questions which this curve answered were {a) whether 

 there was any relation of simultaneity between the absorption and 

 combustion of carbohydrate, and {b) whether there was any likeness 

 between the absorption curves for protein and carbohydrate, for, 

 if so, the conception of rhythmic permeability changes on the part 

 of the cells of the blastodermal blood-vessels would receive support. 



The period of predominance of carbohydrate combustion is be- 

 lie\"ed to be in the first week of development, and from Fig. 252 it 



