DOUBLE MONSTROSITY— HEMIDIDYMUS 25 



end of the united region, the individual right and left recti next rejoin one another within their 

 proper embryos, in which they are continued backwards to their posterior termination (Schmitt 216). 

 The following points relating to anakatadidymi receive mention elsewhere : 

 Frequency of occurrence, p. 7 ; relation to homologous twins in higher animals, p. 7 ; 

 survival after exhaustion of yolk, p. 2 ; orientation of the developing embryonic axes, p. 7. 



CLASS VIII. 



Hemididymi (Mesodidymi and Katadidymi). 



Under these terms are grouped certain forms which are due to similar developmental aberra- 

 tions, and present a characteristically imperfect degree of duplicity. In mesodidymus there is 

 apparent doubling of the middle region of the body, the anterior and posterior ends being single, 

 while katadidymus is marked by apparent posterior duplicity. 



As far back as 1863, Lereboullet (1S/.3), dealing with living material at very early stages, 

 described various examples taken from the pike. Careful descriptions were afterwards given by 

 Oellacher (176) of more advanced stages in Salrno salvelinus, examined both superficially and by the 

 method of serial sections. This author proposed the term mesodidymus as being supplementary to 

 ana- and katadidymus already introduced by Forster. Eauber (202 5) next added a description of 

 very early specimens in Salrno fario. He had previously suggested that Oellacher's specimens 

 should be called hemididymi to indicate the imperfect character of the doubling which they 

 exhibited (200 71). 



Almost from the first, the problem of the origin of hemididymus was closely connected 

 with the theory of concrescence. Lereboullet enunciated this theory, practically though not 

 formally, in the course of his observations on the pike. Oellacher did not consider it needful to 

 adopt the theory in question, which had meantime been definitely formulated by His. Eauber 

 was a supporter of concrescence, which seemed to him to be alike confirmed by, and explanatory of, 

 the hemididymous condition. Later, the experimental work of various observers, notably Kopsch 

 (132-133), Morgan (162), and Sumner (242), settled the major question of concrescence in the 

 negative for fishes (see below, p. 27), and at the same time threw much light on the origin of the 

 abnormalities at present under consideration. These have also received illustration from the study 

 of similar defects in other animals, particularly in the Amphibia. 



Structure. We may begin with a summary for mesodidymus as described by Oellacher (176) 

 in Salmo salvelinus. There is more or less complete doubling throughout a longer or shorter portion 

 of the middle region of the body, while the anterior or posterior ends both externally and internally 

 are perfectly single. The complete doubling involves the mesial organs alone, namely the nervous 

 system, the notochord, the gut, and also in certain cases the liver. On the other hand, all the 

 lateral paired organs, namely muscle plates, Wolffian ducts, auditory organs, and peritoneal cavities, 

 are present only in the total number characteristic of a single embryo. They lie on the outer side of 

 the twin embryonic axes, and are deficient (except for occasional rudiments of the muscle plates) on 

 the inner or adjacent sides. The duplicity of the central nervous system and of the notochord 

 always extends further back and further forward than that of the gut. The length of the hiatus or 

 space between the two components varies greatly. It may only affect the middle of the body (and 

 that not very deeply), or it may affect the whole body from the mid-brain region right back to deep 

 in the tail. In early stages while the yolk-sac is large, the breadth of the space between the 

 components varies proportionately to its length, but in later stages when the yolk is absorbed the 

 twin axes come to he close to one another, and the space in question is reduced to a very narrow slit, 

 the bodies at the same time becoming greatly contorted. 



In the floor of the hiatus, the yolk appears, not with a free surface, but covered by ectoderm, 

 beneath which a thin sheet of mesenchyme containing blood-vessels seems also to occur. At the 

 posterior end of the hiatus there is nearly always to be found a peculiar tubercle, made up of large 

 epidermal cells, and situated just in front of the place of union of the two spinal cords. 



