TERATOGENESIS. 623 



THE SIGNIFICANCE OF THE FOREGOING IN EXPLAINING THE PRODUCTION 

 OF HUMAN MONSTERS. There is, of course, no way to obtain experimental 

 evidence for or against any theory so far as the human subject is concerned. 

 But it is possible to compare the results of experiments on the lower animals 

 with condions found in human embryos. So many malformed human embryos 

 resemble in a general way and often in detail the monsters in the lower forms 

 produced by experimental means that a probable similarity in the causation 

 of them at once suggests itself. The monsters in the lower forms are artifici- 

 ally produced by interfering with the normal course of development of the 

 egg, and by disturbing the normal conditions of nutrition and growth. The 

 disturbing factors are mechanical or chemical, or both. 



According to the recent opinion of Mall, the primary disturbing factors in 

 man are not poisons in the maternal blood, corresponding with chemical agents 

 used in experiments, but the faulty implantation of the ovum in the uterine 

 mucosa. This means that, after the fertilized and segmenting ovum has 

 passed down the oviduct and entered the uterus, it fails to become properly 

 embedded in the mucous membrane. The immediate result is an imperfect 

 formation of the foetal coverings, especially of the chorion. 



The reasons for the faulty implantation are not clear, but they are possibly, 

 even probably, to be found in the condition of the uterus. The most plausible 

 explanation is that some form of endometritis makes the uterine mucosa in- 

 capable of properly adapting itself for the reception of the ovum. 



In the case of the human embryo, such an imperfection in the agency through 

 which it receives its nourishment might be considered in a sense analogous to 

 the external influences that produce monsters in the lower forms. 



Recognizing the fact that a given animal passes through its embryonic 

 stages at a specific rate of development, which is probably dependent upon 

 the rate of oxidation in the protoplasm, Stockard has conducted an extensive 

 series of experiments on animals (Fundulus, or sea-minnow) and analyzed the 

 results with the view of throwing light upon the probable causes of malforma- 

 tions and monsters. His experiments comprised the interruption or slowing 

 of the rate of embryonic development either by temporarily lowering 

 the surrounding temperature and thereby reducing the rate of oxidation in 

 the protoplasm or by cutting off the supply of oxygen. When the tempera- 

 ture of the eggs during the cleavage stages was reduced for a time to 5 or 6 

 Centigrade and then raised to normal it was found that in later development 

 a significant percentage of twins and a number of malformations appeared. 

 Similar results were obtained by directly reducing the supply of oxygen. 

 When exposed to such experimental conditions in later stages of development, 

 the eggs did not produce duplicate monsters. There seemed to be a critical 

 stage at which it was necessary to apply the experimental conditions in order 



