DOUBLE MONSTROSITY STRUCTURE, EARLY STAGES 9 



(4) The fourth mode is illustrated by the examples of longitudinal or parallel union which 

 are referred to on p. 29. 



A notice regarding the prevalence of the different types of double monstrosity in fishes 

 and in other vertebrates is given at the end of the description of hemididymi (p. 28). 



F. STRUCTURE. 



Early Stages of Double Monstrosity in the Salmonidae and in Esox- - p. 9 



Normal Advanced Embryos of Salmo fario p. 11 



Advanced Stages of Double Monstrosity in Salmo fario : 



Class I. - - - - - p. 12 Class VI. - - . - - p. 21 



Class II. - - p. 15 Class VII. - - p. 23 



Class III. - - p. 18 Class VIII. - p. 25 



Class IV. - - p. 19 Class IX. - - p. 29 



Class V. - - p. 20 



Double Montrosity in other Fishes - - p. 30 



EAELY STAGES IN THE SALMONIDAE AND IN ESOX. 



From the anatomical point of view, the stages of chief interest are those in which a large 

 number of structures have become differentiated. Accordingly, the oldest available examples will be 

 selected for detailed study. The anatomy of a normal trout embryo of corresponding age will also 

 be described. But it may be well first to refer briefly to the earliest instances observed, beginning 

 with the Salmonidae. 



Rauber (201), in 1879, described an egg of Salmo salar, which is of normal size and has the 

 blastoderm covering a small area at one pole. On opposite sides of the blastoderm two quite similar 

 centres of embryo formation are appearing. Each shows a normal developing head rudiment. The 

 bodies have not yet begun to form. 



In the following year the same author (202) described seventeen Salmonid ova showing 

 twin embryonic rudiments, and one showing a triple form. See Text-fig. 3 on p. 7, and Text- 

 fig. 5 on p. 36. The whole provides an excellent series illustrating the different distances 

 from one another at which twin embryos may appear, some pairs being exceedingly closely approxi- 

 mated, while others are at directly opposite sides of the blastoderm. Sections were also made by 

 Rauber of several of these monstrosities. 



Kopsch (133 p. 233 Taf. XVII. fig. 21) figures and describes an extremely young stage 

 in the trout. The centres of the two embryonic rudiments are separated from one another by an 

 angular distance of something like 90. 



Schmitt (217} gives an account, with drawings, of two equally young stages in the same 

 species. In the one case the centres are on exactly opposite sides of the blastoderm, while in the 

 other they are almost so (155-165). These two eggs would probably have given rise to twins 

 with union by the yolk-sac alone. 



The next earliest record for the Salmonidae seems to be that of Barbieri (6). It has reference 

 to an egg of Salmo irideus fixed seven days after fertilisation, showing a double embryo 3 mm. 

 long. This specimen is referred to further on p. 29. 



Not much older than the above are two other instances of twin embryos recorded by Schmitt 

 (21 7). They are at the stage when the blastoderm is just completing the overgrowth of the yolk, 

 and they give valuable evidence that the process of overgrowth (as was indicated above) is delayed 

 at, and near, the region where gastrulation began to take place. 



The two specimens which Rauber (200 71) examined by the method of serial sections have next to 

 be mentioned. They are somewhat older than the foregoing. One is an example of union at the level 

 of the optic lobes, and the other of union in the pectoral region. The figures given go some way 



B 



