30 DOUBLE MONSTROSITY— IN VARIOUS FISHES 



his series of anomalous pike embryos is distinguished by the presence of a single broad embryonic 

 rudiment showing more or less complete duplicity of the axial structures especially in front. No 

 doubt this type approaches the anadidymous condition, but following Lereboullet's description one 

 must infer that it will also include examples of parallel union. 



Dohrn's (57) Torpedo embryo, of which a short account is given on pp. 31-32, seems an almost 

 perfect example of the class. Unfortunately details are not available regarding the condition of 

 the anterior and posterior ends of this embryo. 



Mode of Origin. There is little evidence to go upon, but if, as seems certain, the class is 

 really separable from the anadidymi, and therefore justifiable on logical grounds, we must assume 

 that there has been, to begin with, a single centre of gastrulation, and that in connection with 

 this centre there have appeared two sets of axial structures (notochord and nerve cord) lying 

 parallel with one another. Ultimately, posterior union of these axial structures, or entire reduction 

 of one set of them, cannot fail to occur during the progress of growth, since no reserve of cell 

 material is available for the formation of inner or adjacent parts (see p. 6 and p. 27). In 

 the end therefore the condition will become an anadidymous one, but from the disposition of the 

 twin components, the tendency to loss or concealment of duplicity through secondary fusion will 

 have the chance of operating very fully, and Lereboullet's observations show that this actually 

 takes place. 



DOUBLE MONSTROSITY IN OTHER FISHES. 



Blennius. Kathke (199). An anakatadidymous specimen: two separate embryos, one larger 

 than the other, the smaller joined to its neighbour and to the yolk-sac by a short connecting band 

 (quoted from v. Baer 5 89). 



Cyprinus Blicca. v. Baer (5). Two anadidymi: (1) two heads united in neck region and 

 diverging at an angle of 60°; (2) union behind the middle of the body, divergence at 110°. Both 

 specimens were observed 2\ days after fertilisation. 



Scomber. Sutton {'24-5). Mention of double monstrosity in the mackerel. 



Anarrichas. Buckland (33a). A more than doubtful instance, referred to on p. 2. 



Lcnciscus. Bataillon (11) describes fragmentation of ova temporarily exposed to differences of 

 osmotic pressure. This was followed by an attempt at independent development on the part of the 

 fragments. The results, which were experimentally obtained, are of somewhat doubtful nature. 



Girardinus Caudimaculatus. An example of anakatadidymus is recorded by Emeljanov (65) 

 in this species. The twins were united together ventrally behind the second pair of fins by means 

 of cutaneous structures. They were not so large as normal newly hatched embryos, one of them in 

 particular being smaller and less active than its neighbour, to the movements of which it proved a 

 hindrance. They lived for about a week after hatching. 



Petromyzon. Bataillon (1:3) describes a particular group of about a hundred artificially fer- 

 tilised eggs of the lamprey. In 40 per cent, of these the first segmentation furrow — a vertical one — 

 went deeper into the substance of the egg than the later ones, so that the whole mass was to some 

 extent divided into two halves by a vertical constriction. These halves gave rise each to a blastula 

 and gastrula, the blastoporic openings being on the under side. In later development most of the 

 eggs died off, but four managed to survive, twin larvae hatching out from each. 



Experimentally Bataillon (11) succeeded in obtaining imperfect imitations of the above, by 

 exposing lamprey eggs at suitable stages to temporary changes of osmotic pressure. 



Selachii. There are a fair number of records relating to double monstrosity in the cartilaginous 

 fishes. All are examples either of anterior duplicity or of union by the ventral abdominal walls 

 (yolk-sac union). The proportion of the latter to the former is much greater than in the trout, 

 namely, 1 to 2 - 25 in contrast with 1 to 13 (see p. 7). This is what might be looked for, consider- 

 ing the relative sizes of the blastoderms and yolks in the eggs of cartilaginous and osseous fishes. 

 The tendency to primary fusion will operate over a smaller proportion of the whole blastodermic 



