TERATOGENESIS. 625 



factor can reasonably be considered as the only cause in the production of 

 malformations. Granting, however, that both hereditary and external influ- 

 ences are at work in the production of monsters, it is still difficult to determine 

 the separate role of each factor; on the one hand, either influence may appear 

 capable of having produced some given anomaly; on the other, both of them 

 may have been responsible for its appearance. 



The first phase of the theory of external influence was presented three- 

 quarters of a century ago when attempts were made to produce monsters. The 

 experiments led to the formulation of the mechanical theory, which, when applied 

 to human monsters, considers them as the results of mechanical influences upon 

 the embryo, such as the pressure caused by tight lacing or by contractions of the 

 uterus. This theory was gradually transformed into the view that amniotic 

 bands compress or constrict the embryo, thus bringing about malformations. 

 In its turn the latter supposition has recently been criticized and the view sub- 

 stituted that the amniotic adhesions are the results of malformations and not 

 the cause of them (Mall). 



The theory of external influence seems recently to be losing ground in favor 

 of the physico-chemical theory. The latter has gradually been evolved during 

 the course of a great number of experiments on the production of malformations 

 and monsters among the lower forms of animals. It has gained ground because 

 certain definite malformations have been obtained by subjecting the living egg 

 or young embryo to unusual conditions. The experiments consist of interfering 

 in some way with the normal course of development. The interference may be 

 mechanical or chemical, or both, but is always of such a nature as to cause the 

 egg or embryo to develop under unnatural conditions either in an unnatural 

 environment or after having had some of its own substance wholly or partly 

 removed. The results obtained, the strange creatures which develop after 

 such interference, are not infrequently comparable with malformations and 

 monsters found among the higher animals, and they strongly suggest that mal- 

 formations among the higher forms of animal life are the results of similar in- 

 terference with the normal course of development of the egg. 



THE PRODUCTION OF DUPLICATE (OR POLYSOMATOUS) MONSTERS. By 

 shaking sea-urchin ova when in the two-cell stage so that the blastomeres are 

 separated, each blastomere can be made to grow into a whole embryo. De- 

 pending upon the degree of separation, the two embryos will be separate or more 

 or less united forming a double monster. If sea-urchin ova are placed in a 

 mixture of equal parts sea-water and distilled water shortly after fertilization, 

 the cell membranes rupture and part of the protoplasm bulges out. When the 

 ova are replaced in normal sea-water cleavage begins and one of the two primary 

 nuclei wanders into the extruded protoplasm. Each nucleus with its proto- 

 plasm becomes an embryo, and the result is a double monster. If the outflow of 



