134 Anatomy of Symmetrical Double Monstrosities in the Trout. 



(2.) Certain instances of correlation and irregularity in develop- 

 ment. Mode of origin of double monstrosities in the trout. 



The discussion under these heads is based on the evidence brought 

 forward in the descriptive part of the paper. 



(1.) It is shown that at the region of transition in laterally symmetri- 

 cal double monstrosities the notochords are the last structures to unite, 

 while equally primitive structures, both dorsal and ventral to the 

 notochords, viz., the neural axis and the alimentary canal, lose their 

 duplicity earlier. It is further shown that those parts of the neural 

 axis and alimentary canal which are most closely apposed to the noto- 

 chords retain evidence of original duplicity longer than parts which 

 are more remote. The floor and roof of the neural axis and of the 

 alimentary canal are seen to be in marked contrast in this respect. 



Duplicity of the dorsal aorta, of the pronephric glomerulus, of the 

 vertebral cartilages, of the body muscles and of various other struc- 

 tures is correlated with duplicity of the notochord. 



In paired organs the transition from the double to the single condi- 

 tion takes place at the expense of the inner or adjacent elements, which 

 are usually united and reduced in size before they disappear altogether. 

 A list is given of the more important examples of union and reduction 

 in size of adjacent elements in the transitional region, which are 

 mentioned in the descriptive part of the paper. 



From the evidence brought forward it is inferred that fusion has 

 played a not unimportant part in moulding the form of the neural axis 

 and the alimentary tract in the transition region. The union of 

 adjacent paired structures is probably to be explained by the fusion of 

 mesoblastic blastema developing laterally from each of the embryonic 

 axes near the place of convergence and union. 



(2.) The law that union takes place between homologous structures 

 always holds good. Both twins usually contribute equally and 

 symmetrically to the sum of structures in the transitional region. A 

 short list of exceptions to this rule is tabulated, but their paucity 

 and want of importance only serve to make more striking the general 

 symmetry of structure in all the specimens examined. 



With the rarest exceptions, all double monstrosities in the trout are 

 examples either of anterior duplicity or of union by the yolk-sac only. 

 This contrasts very markedly with the types of double monstrosity 

 found in the higher vertebrates, particularly in the birds and mammals. 

 An explanation is suggested which depends on the mode of origin of 

 the primitive streak in osseous fishes and on the manner in which the 

 blastoderm overgrows the yolk mass. 



