274 SALMONID^ OF BRITAIN. 



that these double monsters are formed by a coalescence of two originally distinct 

 embryos, and that the vitellus from which his had been developed had also been 

 doiible, the point of junction being in his opinion indicated by a deep notch at 

 the anterior part of the vitellus. He also thought that certain living alevin 

 monsters upheld his opinion. On the other hand, M. Coste believed that the vitellus 

 was invariably single, and that the circulation in double monsters was common 

 to the two embryos.* Thompson observed a double primitive groove in a single 

 ovum which would probably have developed into a double monster. 



M. Lereboullet's observationsf appeared to demonstrate that double monsters 



had been developed, had likewise been double, the point of junction being indicated by the deep 

 notch in the anterior part of the vitellus, which has been already referred to. 



* M. Coste asserted that the incubation in double monsters was common to the two embryos, 

 that the greater part of the blood which had circulated in the body of one passed into the vessels 

 of the umbilical vesicle (abdominal vein), whence the greater portion of it was carried by the 

 vitelline vein to the auricle of the other embryo, and so on, and that, in accordance with this 

 reciprocal circulation, the contractions of the two ventricles takes place alternately. (M. Quatre- 

 fages believed that he did not find this to be the case in the two instances observed by him.) 



M. Coste maintained that in double monsters there was only a single vitellus and umbilical 

 vesicle, because at whatever age these monsters were examined, the vesicle was invariably simple, 

 a view further supported by the condition of the circulation. That as the external lamina of the 

 umbilical vesicle formed a common abdominal wall, enclosing the vitellus, it was impossible to 

 regard the two embryos as distinct individuals developed at the poles of a double vitellus, and 

 becoming coalescent at a late period of their youth, as they were actually united from the first into 

 a single organism by this membrane, and their subsequent union was effected by the gradual con- 

 traction of this membrane as the vitellus became absorbed. This conjugation was consequently 

 a primordial phenomenon, and of a more intimate nature than one of simple adherence, as was 

 described by M. Geoffroy Saint-Hilaire, wherein two chickens, hatched from two separate yolks 

 contained in one egg, were found to adhere to one another by the belly. 



f He coincided with M. Coste's views, and his experiments made on the eggs of the pike, 

 Esox htclus, showed that the development of the embryo commenced at the moment when the 

 blastoderm had almost completely enclosed the vitellus, by the formation of a triangular tubercle 

 on the blastodermal ridge, and that from this centre the embryonic fillet took its rise. In many 

 cases this ridge of the blastoderm bears two tubercles, from each of which an embryonic fillet 

 was produced, and the further development of these gave rise to double embryos of various kinds. 

 He found that the formation of monsters could be determined at pleasure by placing the eggs in 

 unfavourable conditions for development. He described the formation of several varieties of 

 these double monsters. (1.) In some two tubercles budded out from 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 vertebra^ appeared, the 

 external ones having their ordinary form and dimensions, while the internal gradually became 

 confused, passing from the body of one embryo to that of the other, thus causing the partial 

 amalgamation of the two embryos. In this way a double fish was formed, arising from two 

 primitive germinating points, in'oduced on the blastodermal ridge, so as to become partially 

 joined ; it had therefore two sei^arate bodies with a common tail. 



(2.) In other eggs the blastodermal ridge gave rise to a long and broad fillet terminating 

 anteriorly by two rounded lobes. Tiro parallel furrows appeared in the fillet and soon developed 

 the vertebral divisions, while the anterior lobes acquired a determinate form, and each produced 

 two ocular vesicles, constituting a single body with two distinct heads. In these cases, however, 

 the duplicity was transitory : the two heads soon came into contact, and became fused together in 

 such a manner as to form only a single head. The mode in which this fusion occurred he had 

 been unable to ascertain. In some cases the two heads appeared to remain distinct. 



(3.) Some embryos had a single head, two separate bodies, and one or two tails ; they were 

 thus formed : — The ridge of the blastoderm, which had the form of a gaping button-hole, 

 produced a single cephalic tubercle, but the formative process goes on in the whole circum- 

 ference of the margin, each half of which acquires a chorda dorsalis and a nervous cord, and soon 

 exhibits the divisions of the vertebrte. When the cephalic tubercle was short, and merely gave 

 origin to the true head, each of the two bodies was furnished with two auditory capsules, two 

 pectoral fins, and a heart ; but when this tubercle was more elongated the anterior part of the 

 body was simple and bore two eyes, two auditory capsules and a single heart, and the body 

 terminated posteriorly in two short branches. 



(4.) This organization of the ridge of theblastoderm into a double embryo, seemed to explain 

 the formation of a simple embryo, bearing on the right side of its body a small tubercle directed 

 backwards and terminated by an auditory tubercle and an active heart. Here the resorption of 

 the parts of the body j)osterior to the heart in one of the embryos was considered the reason, 

 while he witnessed the comjDlete disappearance of one of the bodies in the other instance. 



(5.) In another egg the ridge of the blastoderm showed two contiguous tubercles, one of which 

 had the ordinary form of the cephalic tubercle, while the other was smaller and irregular. The 

 first alone acquired a furrow, and gave rise to an embryo, on one side of which the smaller 

 tubercle was borne. 



