METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. 25 



the chick. Dareste regarded alteration of temperature as one of 

 the best methods of inducing teratological development in the 

 chick. According to Dareste, the hen's egg will not develop 

 below 28 C. but Edwards ('02) places the "developmental zero" 

 at 2O-2i C. Dareste found that the embryo dies at 44 or 

 above but Mitrophanow ('oo) would seem to have obtained 

 development at temperatures up to 50. According to Dareste 

 development below 35 or above 39 C. is nearly always abnormal. 

 Fere ('94) in eggs incubated at 40 and 41 C. noted blasto- 

 derms without embryos, defective heads, defective optic vesicles, 

 spina bifida, failure of the somites to form. Kaestner ('95) 

 performed an extensive series of experiments in which eggs after 

 being incubated at normal temperature for various lengths of 

 time were exposed to low temperatures for various periods, and 

 then returned to normal incubation temperature. He found 

 that when such developing eggs are kept too long at the lowered 

 temperature, the time varying with the age of the embryo when 

 exposed, abnormalities were obtained. They occurred most 

 frequently when the temperature was lowered during the first 

 two days of incubation. Disturbances of the head, heart, and 

 brain were noted, and inhibition of such processes as the closure 

 of the medullary folds, elevation of the head fold of the amnion, 

 and union of the heart anlagen. Alsop ('19) incubated eggs at 

 subnormal and supernormal temperatures and examined the 

 embryos after one to three days incubation. At subnormal 

 temperatures (94 to 102), she noted failure of the neural folds 

 to close, thickenings at the primitive knot, abnormal neural 

 tubes often blocked by thickenings particularly in the lumbar 

 region, curvature of the primitive streak. It seems probable to 

 me from inspection of Alsop's figures that the thickenings noted 

 are due to inhibition of the growing point of the primitive 

 streak. At supernormal temperatures (104 to 108 C.) abnor- 

 malities of the brain, particularly of the optic vesicles and mid- 

 brain, were most frequent. Alsop's figures, however, also show 

 inhibition in the primitive streak. In some cases extra somites 

 appeared. Riddle ('23) exposed dove and pigeon embryos to 

 subnormal temperatures for various periods during the first 

 days of development but noted only a small percentage of 



