158 REPRESENTATION OF PROTOMORPHIC ALTERNATION 



the phenomena of ordinary embryogeny ; for we shall have to 

 notice instances of alternation in which they no longer pre- 

 sent themselves, while they reappear at least as an occa- 

 sional abnormality in the reproduction of the higher 

 species. 



As in the former case, we have examples of alternation, 

 where the form immediately developed from the fecun- 

 dated germ originates but a single gemma, which never 

 becomes detached from its matrix ; so in the latter, a single 

 ovum has been observed to originate two distinct axes of 

 embryonic growth. Cases of a double primitive trace of 

 organization have been met with in the bird's egg, by Dr. 

 Allen Thomson and others, and it is probably in some such 

 way that we may most feasibly account for the origin of 

 what are termed " double monsters."* At all events we 

 have in these, as much as in the best marked cases of al- 

 ternation of generations, a production of two more or less 

 typical organisms from a single original germ ; for it is 

 now generally agreed that such monstrosities cannot be well 

 explained on any supposition of the fusion of two inde- 

 pendent embryos. 



This conclusion rests principally on the following con- 

 siderations : 



1. In all such monsters the duplicated parts are con- 

 nected together, and derive their vessels from a common 

 trunk ; we never find a face springing out of the chest, legs 

 implanted on the head, or any such mal-position of parts. 



2. Double monsters form a continuous series, in which 

 the degrees and modes of deviation from singleness gradu- 

 ally increase, and pass, without any abrupt steps, from the 

 addition of a single ill-developed limb to the nearly com- 

 plete formation of two perfect beings ; so that no theory 

 can be tenable that will not account for the simpler as well 



* Edin. Monthly Journ. Med. Science (1844), IV., pp. 479, 568, 639. 

 See also Vrolik's article on Teratology, in Cyc. Anat. and Phys. 



