26 



DOUBLE MONSTROS ITY— HEMID1DYMUS 



Oellacher also observed and described several specimens showing posterior duplicity (kata- 

 didymi). In these specimens the division, instead of being confined to the middle of the body as 

 in mesodidymus, is carried back through the tail region. The posterior part of the body and the 

 tail are accordingly doubled, although the doubling still remains of the same imperfect kind which 

 has been described above as characteristic of mesodidymus. The most complete example of kata- 

 didymus recorded by Oellacher is one in which the duplicity begins as far forward as the region of 

 the optic lobes. Variations of the typical condition are noted, for example the two components 

 may be unequally developed, one of them being short, defective in parts as regards the notoehord 

 and other structures, or even completely interrupted. Or again, small inner muscle plates may be 

 present at various points. 



In the pike, Lereboullet (lJf-3 20 p. 218 et seq.) described thirteen instances of mesodidymus, 

 and two of katadidymus. As regards mesodidymus, his first example and the example which he 

 illustrates in PI. III. figs. 26, 28, are perhaps the most important. The former was observed on 

 the sixth day after fertilisation, and lived till about the fifteenth day, but never hatched out. The 

 fore part of the head was single, but mesial separation began as far forward as the optic lobes, and 

 continued back to the tail, which became normal. The two components were somewhat unequally 

 developed, but each had a notoehord as well as a spinal cord and an outer set of muscle plates. 

 Eemarkable features in this specimen were the presence in each component of a heart and of a small 

 inner otocyst besides the large outer one. The hearts commenced to beat regularly. Inner otocysts 



did not occur in Oellacher's specimens of the trout. He and 

 Eauber suggest that Lereboullet's observation is likely to have 

 been a faulty one. It seems to me, however, that the for- 

 mation of small inner otocysts is just as likely to occur as that 

 of occasional small inner muscle plates. 



The example illustrated by Lereboullet in PI. III. figs. 26, 

 28, is interesting since it shows the presence of a thin mem- 

 brane lying between the divergent components and covering 

 the yolk. The latter, however, still appears through a small 

 round gap in the membrane at the place where overgrowth 

 is just about to be completed. We may compare this witli 

 the covering membrane described by Oellacher, and we must 

 assume that here the advancing blastodermic margin has con- 

 sisted of epiderm only. 



Eauber (202 5 pp. 694-699) described sLx early examples 

 in Salmo fario — all of them evidently katadidymi. One is 

 at a stage sixteen days after fertilisation. Normally at this 

 stage the various regions in brain and body are beginning to 

 be definite, and the outgrowth for the tail is raised up from 

 the surface. The specimen in question shows dehiscence of the two halves of the embryo from 

 the origin of the optic vesicles backwards (see Text-fig. 4 on this page). Almost symmetrical 

 markings occur on the two sides of the gap and indicate corresponding differentiating cerebral 

 structures. Posteriorly, on either side, a small tubercle is seen which the author thinks must 

 represent the separated halves of a tail bud. The hiatus between the two body-halves is bridged 

 over by a thin layer of ectoderm — the menibrana intermedia — resting on the primary entoderm, 

 or yolk with nuclei but without separate cells. The definitive entodermic layer is, of course, 

 awanting. 



Of the other five cases described by Eauber, one also exhibits dehiscence affecting the whole 

 length of the body, but in the rest, the dehiscence affects the middle and posterior parts only. One 

 of these is of particular interest, since it shows a gap interrupting the continuity of the right 

 component. 



Kopsch (132) succeeded in producing two hemididymi in eggs of the trout by applying electrical 



Fig. 4.— After Rauber (202 5. PI. XLI. fig. 

 19). An extreme example of the hemididymous 

 condition (dehiscence, Rauber), in an egg of the 

 trout sixteen days after fertilisation, h, the 

 bending round anteriorly of the germinal thick- 

 ening gr; oc, region of the primitive optic 

 outgrowths ; m, membrana intermedia ; a, 

 blastodermic surface; b t surface of yolk uncovered 

 by blastoderm ; gm, margin of blastoderm. 



