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." 75 



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 aknost complete in some cases but in others 

 incomplete. And upon this fact is based Isadore Geoffrey 

 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." 76 



"Wilhelm Roux: Uber Mosaikarbeit etc. Anat. Hefte, P. 320. 

 Gesamm. Abhandl. II. P. 859. 



"Dareste: Recherches sur la production artificielle des mon- 

 struosites. Paris, Reinwald, 1891. P. 495. 



