364 Harris Hawthorne Wilder 



feet in development.^ It had, however, probably by chance, used one 

 mouth exclusively in suckling and that exercise of functional activity, 

 even during the brief life of the animal, had given a slight twist to the 

 neck so that the feeding head came to be carried as if it were the con- 

 tinuation of the median axis of the body while the head that took no 

 nourishment lay a little to one side. It cannot be doubted that the per- 

 sistence of this treatment through several years would have increased 

 this tendency so that in the adult state a person who knew nothing of 

 the early conditions would have classified the case as that of a lamb with 

 a parasitic head. 



Since, now, in the case of Sauropsida and Mammalia, so much of the 

 development is gone through with before the incident of hatching or 

 birth, it is to be expected that in most cases of primarily symmetrical 

 monsters there will be found secondary differences at least in the internal 

 organs, at the time of birth, and that these differences will be greatest in 

 those systems which are actively functional from an early period, such 

 as the circulatory and digestive systems ; while the least annount of modifi- 

 cation is to be expected in such systems as the skeleton and muscles. 

 That such is actually the case will be seen in the descriptive part of this 

 paper where the numerous modifications in the circulatory system of 

 otherwise perfectly symmetrical monsters will be contrasted with the 

 symmetry of median eyes, formed from two components, where each 

 muscle, nerve, or other part is repeated on the two sides with at least as 

 much faithfulness as in the two sides of a normal bilateral being. In 

 the case of the circulatory system the modifications would naturally 

 affect mainly the heart and the main vessels, while the arteries distrib- 

 uted to symmetrical components would be as regular and symmetrical 

 as the nerves or muscles. [Cf. the cephalic arteries in double-headed 

 monsters, as shown by Miss Bishop.] 



A third cause, or rather a large class of causes, producing secondary 

 deformation of a primarily symmetrical monster, and one likely to 

 induce all sorts of pathological conditions, is the mechanical hindrance 

 to the carrying out of a given plan of development through the inter- 

 position of some organ which cither encroaches upon the space required 

 for sometliing else or actually blocks the way and renders farther develop- 



^This specimen, Teras XY, was one of those used by my pupil, Miss Bishop, 

 in lier paper upon the arteries of dioephalous monsters. The exact equivalence 

 of the two head components is well shown in the equivalence of the arterial 

 supply. 



