No. 1.] ORIGIN OF HUMAN MONSTERS. 31 
circulated at all in any of the embryos. In all of them the 
spaces in the tissues indicate that there is a marked oedema. 
The mesenchyme is increased in amount, the cells being spread 
apart by the fluids between them. Unfortunately for my 
purpose, Bardeen has not examined early stages of his mon- 
sters, nor has Loeb examined with sufficient care the later 
stages. If we keep Knower’s experiments in mind, we may 
think for the present that the changes in circulation in Bar- 
deen’s experiments are primary, and the other changes, like 
irregular development of the nervous system, dropsy and 
hydrocephalus, are secondary, that is, due to the absence of 
the circulation. At any rate, Bardeen’s description of his 
“X-ray toads” corresponds in many respects with those I 
have given of pathological human embryos in earlier publica- 
tions and in Part III of this publication. 
It is also clear that the necrotic changes in the central 
nervous system of these larvz, as well as those in pathological 
human embryos, can be due to deficient nutrition, but in order 
to produce a finished monster the nutrition must not be im- 
paired too much. The heart may be poisoned sotnewhat, 
which in turn may affect the central nervous system, causing | 
histolysis and dissociation there, and the general development 
may be retarded and the embryo deformed, but as soon as the 
heart ceases to function all growth ceases and the embryos 
gradually disintegrate, as has been the case in about 100 speci- 
mens out of my 169. 
In very rare instances human embryos continue to develop 
to the end of pregnancy without a heart. Such a specimen 
must be one embryo of duplicate twins, with a common 
umbilical cord through which it may receive nourishment 
from its healthy brother. Quite early in development its 
circulation must be reversed in the descending aorta, for as 
soon as its heart stops blood enters the body through the 
umbilical arteries, which passes in a reversed direction up 
the descending aorta. Under these conditions all kinds of 
curious monsters develop, ranging from a single head with- 
out a body to a kind of teratoma known sometimes as a 
