1903.] MONSTROSITIES IX FISHES. 19 



cannot exercise the same compelling influence in approximating 

 the growing embryonic axes as it may do in fishes. The question 

 of the origin of the clifierent kinds of double monstrosities in 

 birds and mammals is complicated by other factors and cannot be 

 discussed f villy here ; but the considerations suggested above may 

 throw some light on the fact that pi-actically all double monstros- 

 ities ainong fish with united bodies show anterior duplicity, 

 whilst in mamma,ls and bii'ds there are as many or more cases of 

 posterior duplicity. 



As was indicated previously (p. 16), both subgroups of Class I. 

 exhibit simple lateral union. It may be interesting, in these 

 cases, to compare the behaviour, as regards union, of various 

 mesial and latei^al organs. Of the three primitive axial structures, 

 the notochords are the last to unite, and the alimentary canals the 

 first, while the neural axes are intermediate (pp. 7, 8, 10). It 

 may be taken as a general rule, in monstrosities of this type, that 

 structures and parts of structures which lie nearest the noto- 

 chords retain evidence of duplicity longest. Thus, the optic lobes 

 mentioned on p. 8 have single roof parts, while their basal 

 structures are double ; the composite spinal cord (pp. 9, 12) has 

 additional nerve-roots coming off from its ventral aspect ; there 

 are two air-bladder diverticula in a case where the alimentary 

 canal was single up to the mouth (p. 10), while there is only a 

 single liver in a case where the alimentary canal was double 

 down to the pylorus (p. 14); duplicity of the dorsal aorta is 

 coextensive with duplicity of the notochord, while the heart and 

 pericardium are single (p. 9) ; the cartilages of the neural and 

 hfemal arches are in double sets for many somites in a case 

 where all the branchial cartilages are single (p. 8). 



The slowness with which the notochoi-ds unite may be due in 

 part to their small size and to the natvu^e of their tissue, but is 

 probabl}^ to be referred mainly to their central position and to 

 the fact that they are flanked by the bulky mesoblastic somites, 

 so that 2>rimary fii,sion is deferred as long as possible and secondary 

 fusion is prevented. The early union of the neural axes and of 

 the alimentaiy canals, and the earlier union of their dorsal and 

 ventral walls respectively, may be explained in part by primary 

 fusion, if one remembers that the dorsal wall of the neural axis 

 is formed from the outer edges of the neural groove, and that the 

 ventral wall of the alimentary canal is for a long time incomplete. 

 But such facts as the very marked increase of duplicity in the 

 spinal cord as compared with the medulla oblongata in Class I. 

 [a) and (6) (pp. 9, 13), indicate that, in addition to the primary 

 fusion of concrescence, secondary fusion has played some part in 

 moulding organs at the transitional region. The greater 

 simplicity of the medulla is explained, in part by its greater size 

 as compared with the spinftl coi-d, but chiefly by the fact that 

 the notochords are closer together at their anterior ends (where 

 they are surrounded by the parachordal cartilages) than they are 

 in the cervical region where the median muscular mass serves to 



