INTRODUCTION. 
The present communication is the outcome of a study of 
163 pathological human embryos which I have collected 
during the past fifteen years. The first contribution! which 
I made to this subject included a report of 53, and the second? 
of 20 of these embryos. These two studies are rather 
anatomical in nature and do not consider the causes which 
produce pathological embryos, nor their relation to ordinary 
human monsters. A more careful study of my specimens, 
which have more than doubled in number during the past five 
years, establishes beyond doubt (1) the identity of path- 
ological embryos and small monsters, that is, many of them 
would have developed into real monsters if they had not been 
aborted, and (2) that all of them are developed from normal 
ova due to external influences,—in man to a condition which 
I shall term faulty implantation. 
From the earliest ages in the world’s history the study of 
monsters, and the causes which produce them, has been one 
of the capital problems in anatomy, medicine and natural 
history. Supernatural causes for the production of monsters, 
like the influence of the gods, celestial or diabolical, or lunar 
influences, as expressed in their name, monster or moon-calf, 
held for a long time universal sway and are still believed in 
by many ignorant people. However, the Greek naturalists 
and physicians were inclined to ascribe them to natural causes, 
which belief was gradually displaced in the Dark Ages by the 
one that monsters were hybrids of bestial origin, a theory 
which was finally overthrown in the eighteenth century. 
Equally general has been the theory that maternal impressions 
affect the offspring and convert them into monsters. This 
*Mall, Welch Festschrift, Johns Hopkins Hospital Reports, IX, 1900. 
*Mall, Vaughan Festschrift, Contributions to Medical Research, Ann 
Arbor, 1903. 
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