Headless but Otherwise Normal Monsters 113 
nected by reciprocal actions with the absent parts, and 
therefore is not carried on by the reciprocal action of 
all the parts of the whole, one upon another.” 7 
It is the same with headless monsters as with all 
monsters which lack entire parts of the organism but are 
nevertheless normal in the other parts. Because they 
show likewise that there does not exist any formative 
action exercised by the head, or by other parts, upon 
the rest of the organism. 
While thus the head can be absent in development, 
the presence of certain other parts seems on the con- 
trary to be indispensable in headless omphalosite mon- 
sters: “When one studies,’ writes Dareste, “headless, 
omphalosite monsters comparatively, one notes that the 
trunk is almost complete in some cases but in others 
incomplete. And upon this fact is based Isadore Geoffroy 
Saint Hilaire’s division of headless monsters into three 
different types: the true acephali, in which the thoracic 
region is as well developed as the abdominal region; the 
paracephali, which have only the abdominal region; and 
the mylacephali in which only the sacral region is pres- 
ent. These three types arise through inequalities in the 
development of the cerebro-spinal axis. But how is it 
that the posterior part of this axis is always present, 
while the anterior part is lacking to a greater or less 
extent? Why does not the reverse appear in other cases? 
This depends evidently on some as yet unknown fact 
of embryogeny. For the present we must be content 
with the mere question.” 7° 
Wilhelm Roux: Uber Mosaikarbeit etc. Anat. Hefte, P. 320. 
Gesamm. Abhandl. II. P. 859. 
™Dareste: Recherches sur la production artificielle des mon- 
struosités. Paris, Reinwald, 1801. P. 495. 
