4 CLARKE ON THE DEVELOPMENT OF 



exists, either as a newly formed or as a product of the union of the two individual 

 tendencies, the ability to develop one normal form. Still less probable does this seem 

 when we consider the forms in which duplicity is but very slightly indicated, [as, ^ for 

 example, where there are two thumbs on each hand. In this case one would be forced 

 to believe that these small appendages were the only expression of that strong tendency 

 of each ovum to produce a being like the one from which it had its origin. In other 

 cases where the duplicity is much more complete, as in the Siamese twins, it may be 

 supposed that the embryos did not come in contact until after they were considerably 

 advanced. This would seem somewhat more reasonable. Moreover, if two ova are united 

 intimately and then develop into a monster but slightly duplex, we should expect to find 

 the animal of unusual size. This, however, is very often not the case. In the Ichthyop- 

 sida, and especially in the Teliosts, where many eggs are hatched at the same time, and 

 among which specimens of duplicity are not uncommon, these latter are often, though not 

 always, smaller than their brothers of the same age. This would seem to indicate that, 

 in some instances, duplex monstrosities arose from the union of two eggs, while others 

 came from a single egg. 



The idea that one egg may give rise to two animals, or to a duplex form, has also been 

 often advanced. This theory would account for all such forms of every degree, and this 

 is certainly a strong point in its favor, as there exists a complete series between the 

 extreme former, by supposing that there existed in some eggs an unusual amount of 

 developmental activity, so that, instead of developing in the regular way, it pushed beyond 

 its bounds and formed additional parts. By this theory one can explain all cases of 

 duplicity by supposing a greater or less degree of extra developmental power. A small 

 amount might produce only a secondary thumb, while an extreme amount might give 

 rise to a nearly or quite complete form like itself 



This theory that double monsters originate from one egg has better support than 

 any other. In the first place, as there is a continuous series of these forms from one 

 extreme of duplicity to the other, a theory that shall be satisfactory must explain them 

 all. That is true of this theory. Secondly, these forms are not (so far as I have had a 

 chance to investigate) any larger, and not often so large, as other individuals born at the 

 same time. Were they the product of two eggs this would probably not be the case. 

 Thirdly, all double monsters in which the bodies are sufficiently developed are both 

 invariably of the same sex. 



A very interesting case in connection with this question in Teratology came under 

 my notice in the spring of 1879. I had in my aquarium a large collection of between 

 two and three thousand eggs of Atnhli/stoma punctatum, of which I was studying the 

 development. Examining great numbers of them daily, I chanced to find one in which 

 the medullary folds were nearly completed, but in which the latter had not united at the 

 cephalic end, and in which they appeared so much elevated and rounded at their anterior 

 ends, with well defined instead of ordinary vague outlines, that I kept the egg by itself 

 and watched its development. When first found it was in about the condition represented 

 in plate 1, fig. 1. Figs. 1 and 2 are from memory. The original sketches from life were 

 unwittingly destroyed. Having watched the development and made sketches of the 

 embryo my memory is very clear on the subject; so that, while it is much to be regretted 



