110 EXTERNAL FACTORS III. 6 



embryos were small and abnormal in respect of the medullary 

 tube and amnion; the optic vesicles and cranial flexure were 

 absent, and there were serious disturbances in the area vasculosa, 

 where, though the blood islands were present, the capillaries 

 were either not formed or failed to reach the embryo. No 

 haemoglobin was produced. Embryos exposed at a later stage 

 (four days) nearly all died in two days of asphyxia, the blood 

 being dark red and haemorrhages numerous. That these effects 

 were due not to the reduced pressure but to the want of oxygen 

 was shown by the complete normality of embryos reared in an 

 atmosphere of pure oxygen at the same pressure (except in 

 certain characters always exhibited by such embryos ; see below). 



Similar methods may be employed to demonstrate the neces- 

 sity of oxygen for the Frog's egg, a necessity which is indeed 

 patent to any one who has observed the inferior development, 

 accompanied by spina bifida and open blastopore (Morgan) of 

 the eggs in the middle of a mass of spawn. 



Thus, according to Rauber, development is retarded at a 

 pressure of ^ atmosphere of ordinary air, and the mortality high, 

 while at pressures of \ or \ atmosphere death very rapidly 

 ensues. As a result of four days' exposure to pure hydrogen 

 or nitrogen (ordinary air from which the oxygen had been 

 removed) Samassa observed retarded segmentation, and subse- 

 quently irregularities in development of the type already referred 

 to. Carbon dioxide produced irregular segmentation and death 

 in twenty hours. 



Godlewsld's experiments are perhaps more thorough. The 

 eggs subjected to ordinary air at a greatly reduced pressure 

 (2 mm.), as well as those kept in thoroughly boiled water, 

 segmented but little, and cell-division was confined to the 

 animal hemisphere. In an atmosphere of pure oxygen at the 

 same low pressure, however, development was, in many cases at 

 least, neither retarded nor abnormal. Further experiments with 

 pure oxygen, pure hydrogen, and an atmosphere composed of 

 oxygen and carbon dioxide in equal parts, gave the same result, 

 as the subjoined table shows (Table XIII). It is also clear that 

 the absence of oxygen makes itself felt almost from the begin- 

 ning, while pure oxygen accelerates development. 



