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Show 040 J,A WS OF VAlUATION. CHAI'. XXVI. faces. Whenever two bodies or two heads are united, each bone, muscle, vessel, and nerve on the line of junction _see~s to seek out its fellow, and becomes completely fused w1th It. Lereboullet/ who carefully studied the development of double monsters in fishes, observed in fifteen instances the steps by which two heads gradually became fused into one. In this and other such cases, no one, I presume, supposes that the two already formed heads actually blend together, but t~at the corresponding parts of each head grow in~o one _durmg th_e further progress of development, accompamed as It always JS with incessant absorption and renovation. Double monsters were formerly thought to be formed by ~h~ uni~n ~f two originally distinct embryos developed upon d1stmct vitelh; but now it is admitted that "their production is due to the sponta" neous divarication of the embryonic mass into two halves;" 3 this, however, is effected by different methods. But the belief that double monsters originate from the division of one germ, does not necessarily affect the question of subsequent fusion, or render less true the law of the affinity of homologous parts. The cautious and sagacious J. 1Uiiller,4 when speaking of Janus-like monsters, says, that "without the supposition that some " kinO. of affinity or attraction is exerted between corresponding "parts, unions of this kind are inexphcable." On the otl1er hand, V rolik, and he is followed by othertl, disputes this conclusion, and argues from the existence of a "·hole series of monstrosities, graduating from a perfectly double monster to a mere rudiment of an additional digit, that " an excess of formative " power" is the cause and origin of every monstrous duplicity. That there are two distinct classes of cases, and that parts may be doubled independently of the existence of two embryos, is certain; for a single embryo, or even a single adult animal, may produce doubled organs. Thus Valentin, as quoted by V rolik, injured the caudal extremity of an embryo, and three days afterwards it produced rudiments of a double pelvis and of double hind limbs. 2 'Comptes Rendus,' 1855, pp. 855, 1029. a Carpenter's ' Comp. Phys.,' 1854, p. 480 ; see also Camille Dareste, ' Comptes Rendus,' March 20th, 1865, p. 562. 4 'Elements of Physiology,' Eng. translat., vol. i., 1838, p. 412. With respect to Vrolik, see Todd's 'Cyclop. of Anat. and Phys.,' vol. iv., 1849-52, p. 973. .CHAP. XX\'1. AFFINITY OF HOMOLOGOUS PARTS. 341 Hunter and others have observed lizards with their tails reproduced and doubled. \Vhen Bonnet divided longitudinally the foot of the salamander, several additional digits were occasionally formed. But neither these cases, nor the perfect series from a double monster to an additional digit, seem to me opposed to the belief that corresponding parts have a mutual affinity, and consequently tend to fnse together. A part may be doubled and remain in this state, or the two parts thus formed may afterwards through the law of affinity become blended; or two homologous parts in two separate embryos may, through the same principle, unite and form a single part. The law of the affinity and fusion of similar parts applies to the homologous organs of the same individual animal, as well as to double monsters. Isidore Geoffroy gives a number of instances of two or more digits, of two whole legs, of two kidneys, and of several teeth becoming symmetrically fused together in a more or less perfect manner. Even the two eyes have been known to unite into a single eye, forming a cyclopean monster, as have the two ears, though naturally standing so far apart. As Geoffroy remarks, these facts illustrate in an admirable manner the normal fusion of various organs which during an eai:JY ~mbryon~c period a:·e double, but which afterwards always umte mto a smgle med1an organ. Organs of this nature are generally found in a permanently double condition ·in other members of the same class. These cases of normal fusion appear to me to afford the strongest support in favour of the p_resent law. Adjoining parts which are not homologous sometimes cohere; but 1his cohesion appears to result from mere juxtaposition, and not from mutual affinity. In the vegetable kingdom Moquin-Tandon 5 gives a long list of cases, showing how frequently homologous parts, such as leaves, petals, stamens, and pistils, as well as aggregates of homologous parts, such as buds, flowers, and fruit, become blended .in.to each other with perfect symmetry. It is interesting to examme a compound flower of this nature, formed of exactly d~uble the proper number of sepals, petals, stamens, and pistils, With each whorl of organs circular, and with no trace left of the 5 • 'l'erutulog;e Veg.,' 18:1:1, livre iii. |