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Show 426 DR. B. C. A. WINDLE ON DOUBLE [May 21, amongst those in my collection (a, fig. 6). On the left side of the right fish, which is itself well-formed and normal in every way, and at about tbe site of the pectoral fins, there is a pointed projection, representing a second embryo. This projection is unprovided with branchia and has no mouth, but on its under surface there is a siDgle median round patch of pigment, which, from its identity of appearance with certain other conditions yet to be mentioned, I take to be an ill-developed eye. (b) In this case the second fish, though much smaller than its normal fellow, is recognizable as a fish. It possesses a mouth and a normally formed right eye, that of the left side being represented by a circular patch of pigment like that alluded to in the first case, (c) In this case the head of the parasite has a mouth and is of a shape approximating to the normal, but possesses no eyes nor even pigment patches to represent them. Lereboullet mentions an interesting case (series ii. no. 19), in which he was able to observe two stages in the development of a parasite. He sa}^, " Je mis a part un oeuf (de brochet) age de cinq jours, offrant une large bandelette embryonnaire normale avec son sillon; mais sur le cote droit de cette bandelette et tout pres de sa base, on voyait se detacher du bourrelet blastodermique un tres-petit tubercule, de forme triangulaire. La presence de ce germe accessoire partant du bourrelet blastodermique m'annoncait la production d'un embryon double, ou plutot en raison de la petitesse du tubercule, un embryon muni d'une languette analogue a celles que j'avais vues precedemment. Cependant je ne revis cet ceuf que huit jours plus tard. Le Poisson age de treize jours etait eclos et tres-agile. II paraissait simple et regulierement conforme; mais en l'examinant avec attention, je vis qu'il existait, au niveau de la nageoire pectorale du cote droit, un tubercule a. peine sensible." From this observation it would appear that minor evidences of duplicity may be of more frequent occurrence than would be supposed, but so slight in their nature as to be easily overlooked. Before leaving the subject of the various classes into which these double forms can be divided it may be well to note (two points. In the first place, it should be observed that as yet no such form as that known to teratologists as katadidymus-a form, that is, in which two bodies are connected with a single cephalic extremity-has ever been described : a somewhat remarkable fact. And, lastly, mention should be made of a singular and most anomalous form described by Klaussner, in which two embryos quite divided from one another lay upon the same yolk-sac side by side, but with the cephalic end of one by the caudal end of the other. I have not seen or found in any of the papers to which I have referred any parallel to this case, which is difficult to account for by any of the theories of duplicity now holding the field. III. General Observations. 1. Belative positions of the two members. The commonest position, as has been pointed out by Knoch, is side by side, but |