126 MALL. [VoL. XIX. 
dence in favor of monsters being due to a change in the 
environment, which causes faulty implantation of the ovum, 
thus impairing the nutrition of the embryo, is found in the 
study of the embryo in tubal pregnancy, where 96 per cent 
of them are monstrous. Were the primary trouble in the 
germ, no more pathological ova should be found in tubal than 
in uterine pregnancies. Furthermore, all this is vouched for 
by comparative experimental teratology. 
To be sure, polysomatous, pansomatous and those meroso- 
matous monsters that are due to an arrest of development at a 
very early stage (monopodia) and those variations of an 
hereditary nature (polydactyly, polymastia) and ordinary 
anatomical anomalies, cannot be due to changed environment 
at a stage so late as the fourth week of pregnancy, and some 
of them, like variations in the hands and feet especially, are 
markedly hereditary, and therefore germinal in nature. How- 
ever, this digression is not altogether to the point; the mero- 
somatous monsters, the subject of this report, are due to a 
direct experiment which is equivalent to the mechanical re- 
moval of most of the villi of the chorion. 
The two sets of twins (Nos. 207 and 341) are alike in 
many respects, for each set is contained in a single chorion. 
The degree of development and degeneration is about the 
same for each set. In No. 207, the younger one, the process 
was severe but not of long duration, while in No. 341 the 
cpposite must have been the case. In both sets the organs 
and tissues are well dissociated, showing the usual changes 
so often seen in the embryos studied. When I first took up 
the study of pathological embryos I was inclined to the idea 
that the changes in the chorion were often of a secondary 
nature, but as the specimens became more and more numerous 
and were preserved better and better, which enabled me to 
study them with greater care, this idea had to be abandoned. 
Now it is clear that we are dealing with a simple experiment 
which must bring about the changes in the ovum and embryo 
to make it pathological. The greater number of ordinary 
abortions in the first month consists of ordinary pathological 
