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Show 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 different kinds of double monstrosities in birds and mammals is complicated by other factors and cannot be discussed fully here; but the considerations suggested above may throw some light on the fact that practically all double monstrosities among fish with united bodies show anterior duplicity, whilst in mammals and birds 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 lateral 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 notochords 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 harmal arches are in double sets for many somites in a case where all the branchial cartdages are single (p. 8). The slowness with which the notochords unite may be due in part to their small size and to the nature of their tissue, but is probably to be referred mainly to their central position and to the fact that they are flanked by the bulky mesoblastic somites, so that primary fusion is deferred as long as possible and secondary fusion is prevented. The early union of the neural axes and of the alimentary canals, and the earlier union of their dorsal and ventral walls respectively, m a y 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 {b) (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 spinal cord, 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 2* |
