418 E. I. WERBER 



rods and cones being discernible (fig. 15). At this level the 

 brain has increased in size and presents a very distorted mass 

 of nervous tissue in which cellular areas predominate. Still 

 further posteriorly the buried optic cup of the right side is ob- 

 served to gradually 'reconstitute' itself. It is very large at 

 this level and its structure still betrays evidence of dissociation 

 (ox. fig. 16). The brain increases in size with every successive 

 section (fig. 17). Some parts of it give the impression of 

 retinal tissue, but no certainty can be felt regarding the 

 latter point. 



The duplication of the brain becomes distinct at a level where 

 the (dissociated) part of the larger component is on one side 

 intimately fused with the medulla of the smaller component 

 ('parasite' — fig. 18). Two notochords and two (incomplete) 

 alimentary tracts are also noted on the monster's right and left 

 side respectively (figs. 17 and 18). Sections passing through 

 the larger component's tail show distinctly (fig. 19) that the 

 partial doubling of the central nervous system has come about 

 through a splitting of one anlage by dissociation. 



A clue to the genesis of the described conditions is found in 

 anterior sections. Here the epithelium is dissociated (d.e., 

 fig. 14) in the region of the mouth and in a part of the blasto- 

 derm. The single oral cavity of the double monster is not 

 continuous, but anteriorly partly occluded by the dissociated 

 epithelium of the mouth (d.m., fig. 15) and posteriorly by the 

 dissociated posterior part of the optic cup, while posteriorly 

 to the latter the lumen of the mouth appears as a narrow slit, 

 being 'plugged' by dissociated epithelium. The latter as well 

 as all of the epithelium of the mouth consists of large, vesicular 

 cells. 



Turning, now, to a consideration of the mode of formation 

 of this monster, it would appear improbable that it has arisen 

 from separated blastomeres and subsequent fusion during the 

 formation of the embryonic bodies. The condition of the nerv- 

 ous system described above would seem to indicate clearly 

 that at first one embryonic anlage has existed until probably 

 about the time of formation of the embryonic body when the 

 greater part has apparently been doubled by dissociation. 



