416 HENRY C. TRACY. 



They might possibly be due to the presence of the large yolk 

 sac in the younger stage. The mass of the larva at hatching is 

 considerably less than that of the yolk; the yolk then must act 

 as a "buffer" and the increased CO 2 is "soaked up" from the 

 blood by the yolk substance inside the vitelline net work nearly 

 as rapidly as the blood absorbs it from the water outside. The 

 increase in concentration in the body fluids of the larvae is, 

 therefore, relatively slight at first and increases slowly until the 

 yolk substance approaches equilibrium with the sea water relative 

 to the CO 2 concentration. It is only in the high concentrations 

 that this can take place during the first 10 minute period. In 

 returning to normal sea water the blood begins to absorb the 

 CO 2 back from the yolk and hence the depression period is 

 pronounced. 



At the beginning of the free swimming stage the amount of 

 yolk remaining is very slight, and in the gills active interchange 

 of gaseous substance is taking place directly between the sea 

 water and the body fluids of the larva; the effect of increasing 

 the concentration of CO 2 in the sea water is, therefore, almost 

 instantaneous and hence the stimulation rapidly reaches its 

 maximum, followed quickly by the depression due to effects of 

 excessive CO 2 . The recovery in the normal sea water is rapid 

 on account of the rapid attainment of equilibrium between the 

 inside and outside of the larva through the branchial system. 



It is, however, difficult to understand why it should require 

 so long a time for the yolk substances to take up the CO*. 

 According to the combined record of the newly hatched larvae 

 (Chart XII.) the heightened reaction lasts about 25 minutes, 

 but only 10 minutes in the free swimming (Chart XIII.). 



There are, of course, other differences between the two stages. 

 In the first there is a relative preponderance of undifferentiated 

 tissue, as compared with the later stage. The chemical relations 

 of young tissue must be different from older. The larva? will 

 endure asphyxial conditions better than the adult (see p. 19). 

 Anaerobic respiration is known in the young of other forms. 

 Susceptibility to strychnine is less in young than in adults 

 (Schwartze, '22). Apparently, young tissue has greater adjust- 

 ment capacities to factors affecting a fundamental activity like 

 metabolism than older tissue. 



