434 Harris Hawthorne Wilder 



incomplete!}' separated embryonal anlagen, in short, a symmetrical 

 double monster. A similar fate would later overtake the two other 

 centers, although the resulting double monster would differ in degree 

 from the other, owing to the varying distance of their centers, and finally 

 the two double embryos thus formed, true diplopagi, would fuse with 

 each other, although imsymmetrically. In Eauber's embryo, with three 

 centers, two of them would later form a symmetrical double formation, 

 the third would be independent of this, but would probably still later 

 fuse with it, although unsymmetrically. These last two cases have a 

 special interest in connection with the several known cases of human 

 double monsters produced at the same birth with a normal and separate 

 twin (cf. introduction, p. 357, note 1). 



The last few pages of the paper suggest that the author considers 

 fusion as an important factor in the production of double formations, a 

 view that of late has been treated with much suspicion, and seems to me 

 rather unlikely. Still he refers the cause in all cases back to a very early 

 stage, and suggests the need of studying also in this connection the 

 holoblastic eggs of mammals, although there is for this as yet but a 

 single available specimen, the early sheep's egg of Assheton. He 

 thinks it unsafe to apply to the mammal the conclusions obtained from 

 the egg of the Amphibian, although technically both are holoblastic. 



Experimental teratology, although, perhaps, more than a century 

 old, has but recently begun to yield definite results. Of special bearing 

 upon this paper I may mention the work of two recent experimenters, 

 Spemann and Stocka.rd. 



The first of these authors^ ^ has experimented with the eggs of the 

 Urodele, Triton, and, by ligating them, generally in the two-celled stage, 

 produces at will double-headed monsters, representing different degrees 

 of duplicity, combined or not with cyclopic defects, according to the 

 treatment. If, for example, he ligates the two-celled stage in such a way 

 that the plane of the ligature coincides with that of the first cleavage 

 plane, a monster is produced with two equal heads ; but if the ligature is 

 obliquely placed with reference to this same cleavage plane, one of the 

 componental heads exhibits a greater or less degree of cyclopia. More 

 than this, Spemann has established a definite geometrical rule by which 

 he can predict which of the two heads will be thus defective, for the 



'^Spemann, H. Ueber experiiiieiitell erzeugte Doppelbildungeu mit cyclopi- 

 schem Defekt. Zool. Jahrbiicher, Suppl. VII, 1904, Festscbrift fiir Weisniann, 

 pp. 429-470, 2 pis., 24 Figs. 



