8 DOUBLE MONSTROSITY— DEVELOPMENT, ETC. 



the Salmonidae by Eauber {202), Kopsch (132) and Schmitt (217). Panum (180, p. 309) seems to 

 have been the first to suggest in so many words that His's observations on normal development, 

 which led up to the theory of concrescence, might be employed to explain the characteristic forms 

 of double monster fish. On the experimental side we owe chiefly to Kopsch (130, 133), Morgan 

 (161-2) and Sumner (24-3) the crucial evidence that an embryonic body of full length and with 

 the proper number of segments, but deficient in certain lateral and ventral structures, may develop 

 after section or destruction of the thickened margin of the blastoderm to either side of the 

 embryonic rudiment (see p. 27). The fact that the extension of the blastoderm over the yolk is 

 delayed at, or near, the blastopore has long been known, and recently Kopsch's (133) experimental 

 work has provided us witli exact data as to the progress of this extension in the eggs of the 

 Salmonidae. 



Origin of double and multiple, forms in vertebrates generally. Whatever be the causation, 

 we may recognise, in vertebrates generally, four somewhat different modes of origin, whether for 

 double (and multiple) monstrosities, or for double (and multiple) unioval separate embryos. 



These different modes are: (1) The appearance of two (or more) embryonic rudiments 

 on a single blastoderm (germinal area in the case of the mammalian blastocyst). (2) The 

 presence in the egg of two (or more) separate blastoderms (germinal areas in mammalian 

 blastocyst). (3) Fission or dichotomy on the part of a single embryonic rudiment. (4) Forma- 

 tion of certain axial structures in two parallel sets on a single embryonic rudiment. 



(1) The first mode is universal in fishes, whilst in birds it holds good for the great majority 

 of recorded instances. No doubt the same thing is true for reptiles. As regards mammals, 

 direct observational evidence is awanting, but it is impossible to understand how the majority 

 of double (or multiple) monstrosities can arise in any other way. 



(2) The second mode is possibly illustrated in fishes by the unique example from Klaussner 

 referred to above. It does not appear to be excessively rare in birds. Thus Dareste (53, 

 PI. I. fig. 4) figures two young blastoderms lying close together which have arisen from the 

 development of two separate cicatriculae. In the succeeding figure the same author shows two 

 blastoderms at a considerable distance from one another, each exhibiting traces of a degenerating 

 embryo. Besides these, the first and fourth examples of triplicity in the chick quoted from 

 Dareste (see p. 37 of this work) are marked by the presence of two separate blastoderms. 

 But perhaps the best example in the chick is a seven days' embryo figured by Panum, which 

 shows two well-developed embryos a considerable distance apart from one another, each 

 surrounded by its own vascular area. Schwalbe reproduces this figure (222, II. p. 37), and 

 on the same page he illustrates also a six-day incubated egg (described by C. W. Wolff) with 

 two quite separate embryos on a single large vascular area. He also states on Panum's authority 

 that Fabricius ab Acquapendente had observed two cicatriculae on the yolk of an unincubated 

 egg. It should be added that Wolff's specimen only doubtfully indicates original doubling of 

 the blastoderm. 



In reptiles we have the Troindonotus example figured by Wetzel (see p. 37 of this work), 

 in which there are three centres of segmentation on the yolk, and one of the centres is itself 

 double. Grundmann also has described an egg of Laccrta with a double blastoderm (quoted 

 from Schwalbe, 222, II. p. 29). 



In mammals (see p. 38) the presence of more than one germinal area on the blastocyst 

 is splendidly illustrated by the edentate Talusia in which plurality is normal, while Assheton's 

 observation shows that doubling of the germinal area can also occur in the blastocyst of the 

 sheep, a mammal the development of which proceeds normally on more ordinary lines. 



(3) The third mode accounts for certain examples of duplicity (chiefly posterior) in birds. 

 There is every reason to believe that it occurs in a similar fashion among mammals as well, 

 but the full discussion of this subject falls outside the scope of our work. In fishes the mode in 

 question is represented only by the peculiar and imperfect doubling characteristic of the 

 hemididymous condition (p. 25). 



