357 



wholly inexplicable, not only how the whole of the second germ, 

 with the exception of the small duplex part, should have so 

 entirely disappeared ; but, also, how the local relations of the 

 duplicate parts should always be so apposite, that a finger is 

 always connected with a finger, a head with a head, &c. 



On the other hand, unless we admit some nevv power in em- 

 bryology, there would be no reason why, in these cases, the 

 supernumerary parts should not as often be removed from their 

 analogues, as be with them. Thus, in this fusion of two germs, 

 we might well expect a toe to be found on the abdomen, or a 

 finger on the head — a condition never met with as far as I am 

 aware. But the argument of the most weight against this view, 

 is, that there being two germs, there must, of course, be two 

 sets of embryonic membranes, and it is inconceivable how such 

 coalescence should have taken place, especially in the instances 

 of the duplicity of the extremities, or any of their parts, — organs 

 which are formed at a late period of the embryonic development, 



In the case of double-yolked eggs, twins may be produced, 

 but all experiments with eggs show that the formation of double 

 monsters, with chicks, is from single-yolked eggs. 



Thus, Dr. Allen Thompson* has described and figured a hen's 

 egg, with two primitive grooves upon one yolk, and in one germ- 

 inal membrane or blastodermic vesicle. This, when fully de- 

 veloped, would, undoubtedly, have produced a double monster, t 



On the other hand, the hypothesis that this duplicity is due to 

 a segmentation of one germ at the parts in question, has for its 

 support the general conditions of development. 



On the supposition, that in the earliest conditions of embryonic 

 life, there occurs from, perhaps, an abnormal nutrition, a division 

 of a certain portion of the germ, say, for instance, that of the 



* London and Edinburg Monthly Jour. July, 1844. 



t It is true that with the Turbellaria many embryos are said to be developed in 

 a single egg, (See Siebold, Verhan'dl. d. Akad. in Berlin, 1841, p. 83); but in this 

 case I regard the so-called egg as rather an egg-capsule, investing many true 

 eggs. This view would be sustained by some phenomena observed with Mol- 

 lusks. Thus Miiller (Miiller's Arch. 1852, p. 1.) has seen the ovarian capsule of 

 a MoUusk burst and discharge from fifteen to twenty eggs, which subsequently 

 grouped together, and were invested by a special capsular membrane. 



