Development of certain Amphibian Embryos. 105 



Bufo lentiginosus. — Only a small percentage developed at all 

 and none beyond early cleavage. 



Hyla versicolor and Hyla pickeringii. — A few went through 

 cleavage but none beyond. 



Rana pipiens (?). — Mortality was very large during early 

 stages but perhaps 5 per cent, developed until the embryos were 

 considerably differentiated and about 3 mm. in length. None 

 developed further. In two of the ponds observed all the eggs and 

 embryos of this species died much as those placed in CO2 water 

 in the laboratory. These were ponds practically without aquatic 

 plants and containing great quantities of decaying leaves and 

 other plant debris and therefore doubtless had much CO2 in the 

 water. 



Rana sylvatica. — The development was apparently at the 

 normal rate. In many cases the larvae hatched, but though 

 active for a day or two and clinging to the jelly mass and sides 

 of the jars in the usual fashion took no food and developed no 

 further. All died within a few days. In other jars where con- 

 siderable masses of the eggs were placed and where large numbers 

 died before hatching the larvae in the interior of the masses and 

 thus least exposed to the C0 2 water survived longest. 



Often the eggs of the inner portions of the egg masses of Rana 

 sylvatica and Amby stoma tigrinum (all of which were otherwise 

 developing normally) show a retarded development, progressively 

 the more so the farther the eggs are from the periphery of the mass, 

 due to insufficient oxygen. In one egg mass of Amby stoma the 

 exterior embryos of the mass were 10.2 to 11 mm. long and almost 

 ready to hatch, while at the other extreme those deepest within 

 the mass were only 3.6 mm. long and still in the late neural groove 

 stage. In most cases in the ponds where the Amby stoma eggs 

 are laid, as well as in jars in the laboratory, a portion of the 

 interior of the masses die before hatching. In one pond near 

 the laboratory protected from wind and containing very quiet 

 and poorly oxygenated water perhaps 90 to 95 per cent, of the 

 eggs die each year. That insufficient oxygen is the cause was indi- 

 cated by the low percentage mortality in a few masses left almost 

 entirely exposed to the air by the lowering of the level of the pond. 

 These conditions were imitated in the laboratory with the result 



