On Double Monstrosity in Fishes. 49 



centre the embryonic fillet takes its rise. In many cases the 

 ridge of the blastoderm bears two tubercles, from each of 

 which an embryonic fillet is produced, and the further develop- 

 ment of these gives rise to double embryos of various kinds. It 

 appears from a statement of M. Lereboullet, that the formation 

 of these monsters may be determined at pleasure by placing the 

 eggs in unfavourable conditions for development. 



M. Lereboullet describes the formation of several varieties of 

 these double monsters. 1. In some instances two tubercles were 

 produced on the margin of the blastoderm, from each of which 

 proceeded a fillet furnished with a dorsal furrow, forming two 

 embryos adhering to the marginal ridge. Soon afterwards the 

 divisions of the vertebrae made their appearance, the external 

 ones having their ordinary form and dimensions, whilst the in- 

 ternal gradually became confounded, passing from the body of 

 one embryo to that of the other, thus causing the partial amal- 

 gamation of the two embryos. In this manner a double fish is 

 formed, arising from two primitive germinating points produced 

 upon the blastodermal ridge, so as to become partially joined ; it 

 has therefore two separate bodies mth a common tail. A spe- 

 cimen of this description, in which the fusion of the double em- 

 bryo had extended to about the middle of the body, lived for 

 four days after exclusion from the egg. 



2. In other eggs the blastodermal ridge gave rise to a long 

 and broad fillet terminated anteriorly by two rounded lobes. 

 Two parallel furrows made their appearance in the fillet, and 

 soon exhibited the vertebral divisions, whilst the anterior lobes 

 acquired a determinate form, and each produced two ocular vesi- 

 cles, constituting an embryo with a single body and two distinct 

 heads. In these cases however the duplicity was transitory ; the 

 two heads soon came in contact, and became soldered together 

 in such a manner as to form only a single head. j\I. Lereboullet 

 has observed these phsenoraena in about fifteen instances, but 

 could never ascertain in what manner the fusion of the two 

 heads was effected. In some cases the two heads appear to 

 remain distinct. 



3. One of the most remarkable monstrosities is that presented 

 by some embryos with a single head, two separate bodies, and 

 one or two tails. These are formed in the following manner : — 

 the ridge of the blastoderm, which has the form of a gaping 

 button-hole, produces a single cephalic tubercle, but the forma- 

 tive process goes on in the whole circumference of the margin, 

 each half of which acquires a chorda dorsalis and a nervous cord, 

 and soon exhibits the divisions of the vertebrae. When the 

 cephalic tubercle is short and only gives rise to the true head, 

 each of the two bodies is furnished with two auditory capsules, 



Ann. ^ Mag. N. Hist. Ser. 2. Vol. xvi. 4 



