60 On Double Monstrosity in Fishes, 



two pectoral fins and a heart ; but when this tubercle is more 

 elongated, the anterior part of the body is simple, and bears two 

 eyes, two auditory capsules and a single heart, and the body is 

 terminated posteriorly by two short branches. 



4. This organization of the ridge of the blastoderm into a 

 double embryo seems to explain an extremely curious form 

 which only occurred once in M. Lereboullet's observations. This 

 consisted of a simple embryo, bearing on the right side of its 

 body a small tubercle, directed backwards, and terminated by an 

 auditory capsule and an active heart. The production of this 

 form is accounted for by the resorption of the parts of the body 

 posterior to the heart in one of the embryos, and M. Lereboullet 

 has witnessed the complete disappearance of one of the bodies 

 in other instances. 



5. In another egg the ridge of the blastoderm exhibited two 

 contiguous tubercles, of which one had the ordinary form of the 

 cephalic tubercle, whilst the other was smaller and irregular. 

 The former alone acquired a fuiTow and gave rise to an embryo, 

 on one side of which the smaller tubercle was borne. 



6. Occasionally bodies furnished with three heads make their 

 appearance. One of these is described by M. Lereboullet. It 

 was a double embryo, composed of two bodies united behind, 

 but quite free in front. One of these bodies was of the normal 

 form; the other bore two heads, of which that on the left was of 

 the normal form and furnished with two eyes, whilst that on the 

 right only bore the right eye, the union of the two heads being 

 efi'ected at the point where the left eye ought to have been. This 

 singular embryo was still in the egg when described ; it had two 

 hearts, one common to the two principal bodies, situated at their 

 bifurcation, the other placed in the angle of union of the two 

 heads. 



M. Lereboullet explains the formation of this anomalous crea- 

 ture in the following way. He considers that two fillets have 

 been formed, one of which has been terminated by two cephalic 

 lobes and acquired two furrows (as described under 2.), whilst 

 the other has remained simple. These two embryos have then 

 united posteriorly (as under 1.), producing an embryo with one 

 tail, two bodies and three heads. 



7. An extraordinary result was obtained when the develop- 

 ment of the eggs was retarded by a considerable diminution of 

 temperature. In this case the ridge of the blastoderm pro- 

 duced no embryo, but contracted gradually like the opening of a 

 bag, its substance becoming condensed and forming a mammil- 

 lated tubercle projecting from the surface of the vitellus. This 

 tubercle continued living, rose more and more from the surface, 

 acquired a Ungulate form, and at last constituted an elon- 



