The Morphology of Cosinobia 363 



fetu), and I would wish to remain noncommittal for the present in 

 regard to the majority of dermoid cysts and other embryomata where 

 the parasite is too amorphous to apply the test of homologous union. 

 HoAvever, excepting all these doubtful cases there still remains a large 

 class in which the lesser component is properly related to the greater to 

 constitute with it a primarily symmetrical diplopage, though secondarily 

 deformed. 



A second cause of deformity, at least as regards bilateral symmetry or 

 equality of components, and one which is especially operative in assist- 

 ing in the secondary deformation of a diplopage, is found in the striving 

 among the parts during growth for the best physiological efficiency. In 

 a vertebrate embryo certain of the organs, especially those of circulation, 

 and, to a lesser extent, digestion, are physiologically active from a very 

 early period of development. The former, for example, is early called 

 upon to solve certain mechanical problems connected with the trans- 

 portation of the blood, and although much of the general arrangement 

 of these parts is probably inherent in the germ, the details are mainly 

 left to the exigencies of the particular eases, as is shown by the great 

 amount of individual variation in the adult of a given species, especially 

 in the smaller, later appearing vessels. Now in those diplopagi in which 

 the heart is represented by two separate components, which yet form 

 parts of one system, the problem is presented in a more definite way 

 than in the case of two rival blood-vessels that supply the same part, 

 since there are here two pulsating organs to direct one circulation. It is 

 inevitable in such a case that one of the hearts should early become a 

 little stronger than the other and gain either a control of vessels beyond 

 the median line separating the two components or else secure for its 

 vessels a little more of the blood; in either case the result would soon 

 show in a lack of symmetry between the two components or between 

 corresponding parts, although at first the two components were exactly 

 equal. This point will be brought out farther on by a comparison of 

 the circulation in several cases of the gi'oup known as "Janus" monsters, 

 where the asynmietry in the circulatory system is plainly of a secondary 

 nature and due to the causes above outlined. As an instance of second- 

 ary asymmetry due to the rivalry among digestive systems I may cite 

 the case of a two-headed lamb (Teras XV of my collection), which 

 had come to full term and lived and fed for perhaps four weeks. Aside 

 from the doubling of the head it appeared absolutely normal, i. e., 

 single, and tlie two heads were exact duplicates, of each other and per- 



