300 ARCH E. COLE 



green if the stratum was very thin. Shortly after this, Lankester 

 ('67) obtained the characteristic absorption spectrum of haemo- 

 globin from chironomid blood. The larvae of C. tentans is 

 richly supplied with this pigment, giving a bright scarlet color 

 to the animal. The blood, then, b}^ virtue of its contained haemo- 

 globin has exceptional capacity for taking up oxygen dissolved 

 in the surrounding water and for transporting it throughout the 

 body. But during the stagnation period in Lake Mendota the 

 supply of oxygen cannot be the same as during the preceding 

 period when there is an abundance of dissolved oxygen. The 

 oxygen has been all used up and an excess of carbon dioxide has 

 taken its place. 



This is well pointed out in the tables given by Birge and Juday 

 ('11) for the year 1906. On April 20th the oxygen content of the 

 water in Lake Mendota at a depth of 22 meters was 7.5 cc. per 

 liter, and 8 cc. per liter at the surface. June 11th showed 1.8 

 cc. per liter at 22 meters in contrast to 6.3 cc. per liter at the 

 surface. On July 10th there was no detectable oxygen at 22 meters, 

 a trace at 20 meters, 0.1 cc. per liter at 18 meters and 7 cc. per 

 liter at the surface. As the oxygen was progressively used up 

 at higher levels, the oxygen-free stratum increased in thickness 

 until, on September 30th, no oxygen was detected between a 

 depth of 15 meters and the bottom. Following this time the 

 upper surface of the oxygen-free belt dropped very slowly, due 

 mainly to the decrease in temperature of the surface water, 

 which allowed the heavy fall winds to circulate the water to 

 greater depths. After the fall overturn, which came between 

 October 8th and October 11th, there was 5.5 cc. of oxygen per 

 liter at 22 meters. Between July 10th and October 8th no oxygen 

 could be detected in the water at a depth of 22 meters, and for 

 a somewhat shorter period, August 24th to October 8th, there 

 was none at the 18-meter level. The work of Birge and Juday, 

 extending through a period of over a dozen years, leaves no 

 doubt that this is of annual occurrence. Titrations to deter- 

 mine the oxygen content of the deeper water of the lake were 

 also made by the writer from time to time while these experiments 

 were being carried on. 



