250 CHARLES R. STOCKARD 



access of oxygen from the surrounding air. The one element es- 

 sential for development of these eggs, not yet provided, is a con- 

 stant high temperature. The reptiles are largely confined to 

 warm regions and deposit their eggs during the hottest periods 

 of the year in sand or other heated places, and in this way the 

 proper temperature is usually provided. The birds, however, 

 with the extremely high temperature of their own bodies, supply 

 in a more definite way the proper amount of heat for the incuba- 

 tion of their embryos. Lack of moisture and oxygen very rarely 

 causes the death or abnormal development of the eggs of reptiles 

 and birds. But failure to maintain a uniform temperature and 

 unfavorable degrees of heat and cold are the chief causes for 

 embryonic mortality and deformity in these animals. 



The mammals have advanced a step further in perfecting a con- 

 trolled developmental environment. The internal development 

 of the embryo not only insures a properly moist condition, but 

 the high temperature of the maternal body is sufficiently uniform 

 never to cause interruption of the normal progress of develop- 

 ment. The supply of oxygen is derived from the blood of the 

 mother through the placental circulation, and this is the one 

 element in the mammalian developmental environment which 

 most frequently becomes deranged. Faulty placentation cuts 

 down the supply of oxygen to the mammalian embryo and lowers 

 its rate of development, producing as a result prenatal death and 

 all varieties of malformation. Yet we may well believe that the 

 long and highly complex development of the mammalian embryo 

 could not take place unless it was protected by a fairly well 

 regulated environment. Abnormal development in the embryos 

 of birds may very rarely result in nature from poor ventilation on 

 account of a coated egg shell, but more frequently it results from 

 failure to maintain a uniform temperature. While in mammals 

 the temperature changes are eliminated by the internal mode of 

 development, the one great danger to normal development still 

 not completely controlled is the chance of a low oxygen supply 

 brought about by a delayed or poor implantation of the pla- 

 centa. The great majority of monsters in mammals are very 

 probably due to an insufficient oxygen supply during develop- 

 ment, and this results as a rule from faulty placentation. 



