A DOUBLE-HEADED VERTEBRATE. 5 



that the original sketches are gone, I feel that the two given are accurate in all the 

 essential points. I have also given two figures from life of normal development at the 

 same stages for comparison. As the medullary folds closed in, they failed to unite along 

 a certain part of the thickened cephalic end. Then each free portion of the medullary 

 folds developed a perfect head, which at first partly united, gradually became more so, 

 until they were connected throughout their entire lengths. Posterior to the heads, 

 however, there are no indications of duplicity. There is but one body with the regular 

 appendages and cloaca. One of the heads, the right, is larger than the left, and somewhat 

 more in a direct line with the body than the smaller head of the left side. Each head has 

 a pair of well-formed eyes and a mouth. The branchiae of the right head are quite 

 normal ; those of the right side of the left head are unusually small and crowded down by 

 the branchiae of the left side of the right head ; the branchiae of the left side of the left 

 head are abnormally large, sufficiently so to make good the want of size of those of the 

 right side. The right head is much nearer the plane of the body than the left, as will 

 readily be seen from the position of the eyes and mouth. The smaller head, however, is 

 so twisted, or rotated, that its left side is in about the same plane as the ventral side of 

 the body. Having reached the stage indicated in figs. 5 and 6, the animal died. 



The interesting point in this observation lies in the fact that a two-headed monster, with 

 one regularly symmetrical body, was developed from one egg, and that the anterior 

 portion of each medullary fold gave origin to a head. 



It is a point in favor of the theory of a tendency of singleness toward duplicity ; that 

 is of one egg having a tendency to develop into two animals, more or less completely. 



It is quite surprising to find that the portion of each medullary fold which ordinarily 

 gives rise to a definite half of the head, with its sense organs and appendages, should, in 

 this case, have developed a perfect head with paired eyes and ears and branchiae. 



EXPLANATION OF PLATE. 



Figs. 1-4. All the figures enlarged to thirty diameters. 



Fig. 1. Dorsal view of the earliest stage in which the embryo of the monstrous form was observed. 

 A, the space separating anterior ends of medullary folds, and across which they never united; m/, medullary 

 folds; mp, medullary plate; mr, medullary groove. 



Pig. 2. A later stage of the same embryo. A indicates the region along which the folds remained separate. 

 Other letters as before. Figs. 1 and 2 are from memory ; see above. 



Fig. 3. An embryo of same stage as fig. 1 ; but developing normally. Letters as before. Here the medul- 

 lary folds have united anteriorly. From life. 



Fig. 4. An embryo in corresponding stage with tig. 2, showing normal development. In this there is no 

 separation between the cephalic portions of the medullary folds. Letters as before. From life. 



