10 DOUBLE MONSTROSITY— STRUCTURE, EARLY STAGES 



towards illustrating the behaviour of the more important organs (central nervous system, notoehord, 

 and Wolffian ducts) at the region of transition from the double to the single condition. 



Of about the same age is the single specimen sectioned and described by Moser {165). The 

 two central canals of the spinal cords unite near the fourth body somite, but the notochords only 

 come together at the level of the vent. Inner or adjacent auditory organs are present, but reduced 

 in size, and enclosed withiii the same cartilaginous capsule. The pronephros is peculiar, having three 

 much reduced pronephric chambers and no proper glomerular tuft. It gives origin to three 

 Wolffian ducts. Moser's paper also contains a discussion regarding the origin of double monstrosity 

 in the light of recent experimental work bearing on the question of concrescence. 



Still later stages are dealt with by Schmitt (21 6), and by the author (76-77). In the specimens 

 which they describe, practically all the organs and structures found in the adult, except the osseous 

 skeletal parts, have already made their appearance. Schmitt's work covers only union by the tail 

 and extreme posterior part of the body, and will be referred to later in connection with the anatomy 

 of these types. 



Esox. The eggs of the pike equally with those of the trout and salmon have provided classical 

 material for the study of double monstrosities. The first account of the growth from day to day of 

 a developing double fish was given by Valentin (263) from observations on an example which he 

 watched from the fifth to the thirteenth day. To Lereboullet's work on the same species we owe the 

 fullest investigation which has yet been made regarding the occurrence, and mode of growth, of 

 double monsters in any species. Out of a total of over two hundred thousand eggs examined, he 

 obtained rather more than two hundred double monsters, very many of which he not only observed 

 at an extremely early stage, but also followed in development till their final form became established. 

 A seventy-two hours' stage in the pike was also figured and described by Eauber (202 6 Taf. IX. 

 fig. 23), while two exceedingly early examples are illustrated by Klaussner (123 Taf. I, figs. 1, 2). 



Lereboullet (H-3 20 254 ct scq.) classified the different sequences observed by him in the 

 growth of anomalous forms into the following six types : 



1. Instances in which there are, to begin with, two separate embryonic rudiments. These tend 

 to unite posteriorly, the resulting forms having either separate heads and bodies, or separate heads 

 with united bodies. 



2. Instances in which there is one broad embryonic rudiment showing more or less extensive 

 doubling of the anterior axial structures. Later, by fusion and disappearance of inner elements, 

 evidence of duplicity may disappear almost completely. Or one of the sets of structures may 

 become atrophic. 



3. One instance of a triple monstrosity. There are two bodies, one of which carries a pair of 

 closely joined heads. An even younger triple Salmonid specimen has been described by Eauber 

 (see p. 36). 



4. Instances of the type afterwards called mesodidymus by Oellacher (176), in which the 

 head and tail are single but the body is double. Each component tends to be defective as regards 

 its inner or adjacent elements (p. 26). 



5. Instances of thinness and slenderness of the embryonic rudiment followed later by defective 

 formation of various organs, notably of the brain and organs of sense. 



6. Instances in which no proper embryonic rudiment is formed at all. The blastoderm, 

 however, succeeds in covering the yolk mass, and then a small tubercle makes its appearance, which 

 represents the tail of the fish. 



In his explanation of types 4 and 6, Lereboullet comes very near to enunciating a definite 

 theory of concrescence. 



Lereboullet's youngest example dated back to fifty-three hours after fertilisation, and showed 

 two embryonic rudiments very close to one another, and imited together at their posterior ends. 

 A medullary groove was present in each. His next earliest observation had reference to an egg 

 fifty-four hours after fertilisation. Here he described the embryonic rudiment as being single but 

 broader than normal, and showing two parallel medullary grooves separated slightly from one 



