SECT. 6] OF THE EMBRYO 9^7 



this was an unfortunate substance to choose as a representative of 

 the carbohydrates, in view of the phenomenon of the transitory Hver^. 

 When I calculated this ratio on the basis of analyses of total carbo- 

 hydrate, a definite decrease appeared, just as was expected. The 

 bundle of curves which the plots of the ratios against the age 

 give is shown in Fig. 249, and it is seen that they all pass downwards 

 together. 



Some of the minor points in this graph are worth considering. The 

 decHne in the ash/organic substance ratio, for instance, reminds one 

 of the similar dechne in the case of the frog embryo (see Fig. 230), 

 and of Spek's suggestion that the velocity of cleavage of cells may 

 depend, at any rate partially, on the concentration of electrolytes 

 in which they find themselves. Perhaps we catch a glimpse here of 

 a mechanism which controls cleavage velocity, for if by some means 

 it were arranged that the ash-content of an embryo should fall with 

 age, then the other factor might fall likewise, and the growth-rate 

 would follow suit. 



6-9. Absorption-mechanisms and Absorption-intensity 



So far we have been considering the constitution of the embryo 

 and how it changes with age, but the next question concerns the 

 relative intensity of absorption at different times during its develop- 

 ment. Up to the present time it has only been possible to make a 

 start in this direction with the avian embryo, although morphologically 

 a good deal is known about the various methods of absorption of the 

 materials stored in the egg, and before considering the absorption 

 intensity of the chick, something may be said about the absorption 

 processes of other embryos. The principal treatment of this subject 

 is that of Peter. It is evident that a fundamental distinction will here 

 arise between holoblastic and meroblastic eggs, for if cleavage is 

 complete and the whole egg divides into equal or nearly equal parts 

 from the very beginning, the yolk will also get divided more or less 

 equally, and being there already will not require any special means 

 of transport into the constituent parts of the embryo. On the other 

 hand, where cleavage is partial, and the vegetal pole of the egg does 

 not divide at all, or where, as in the case of birds, 90 per cent, of 

 the egg is vegetal pole, then mechanisms of various degrees of com- 



^ See Section 8-5. 



