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Show 122 THE DESCENT OF 1\IA~. . P.An.T I. shewn by almost all boys in climbing trees ; and this again reminds us how lambs and kids, originally alpine animals, delight to frisk on any hillock, however small. Reversion.- Many of the cases to be here given might have been introduced under the last heading. Whenever a structure is arrested in its development, but still continues growing until it closely resembles a corresponding structure in some lower and adult member of the same group, we may in one sense consider it as a case of reversion. The lower members in a group give us some idea how the common progenitor of the group was probably constructed; and it is hardly credible that a part arrested at an early phase of embryonic development should be enabled to continue growing so as ultimately to perform its proper function, unless it had .acquired this power of continued growth during some earlier state of existence, when the present exceptional or arrested structure was normal. The simple brain of a microcephalous idiot, in as far as it resembles that of an ape, may in this sense be said to offer a case of reversion. There are other cases which come more strictly under our present heading of reversion. Certain structures, regularly occurring in the lower members of the group to which man belongs, occasionally make their appearance in him, though not found in the normal human embryo; or, if present in the normal human ·embryo, they become developed in an abnormal manner, though this manner of development is proper to the lower members of the same group. These remarks will be rendered clearer by the following illustrations. In various mammals the uterus graduates from a dou?le organ with two distinct orifices and two passages, a~ m the marsupials, into a single organ, showing no s1gns of doubleness except a slight internal fold, as in CHAP. IV. l\I.ANNER OF DEVELOPMENT. 123 the higher apes and man. The rodents exhibit a perfect series of gradations between these two extreme states. In all mammals the uterus is developed from two simple primitive tubes, the inferior portions of which form the cornua; and it is in the words of Dr. Farre "by the coalescence of the two cornua ~t "their lower extremities that the body of the uterus IS "formed in man; while in those animals in which no "middle portion or body exists, the cornua remain un" united. As the development of the uterus proceeds, "the two cornua become gradually shorter, until at "length they are lost, or, as it were, absorbed into the "body of the uterus." T~e angles .of t?e uterus are still produced into cornua, even so h1gh m the scale as in the lower apes, and their allies the lemurs. Now in women anomalous cases are not very infrequent, in which the mature uterus is furnished with cornua, or is partially divided into two organs; and such cases, according to Owen, repeat " the grade of con" centrative development," attained by certain rodents. Here perhaps we have an instance of a simple arrest of embryonic development, with subsequent growth and perfect functional development, for either side of the pu.rtially double uterus is capable of performing the proper office of gestation. In other and rarer cases, two distinct uterine cavities are formed, each having its proper orifice and passage.35 No such stage is passed through during the ordinary development of the embryo, and it is difficult to believe, though perhaps not impossible, that the two simple, minute, primitive tubes could know how (if such an expression may be used) to 3 ~ See Dr. A. Farre's well-known article in the 'Oyclop. of Anat. and Phys.' vol. v. 1859, p. 642. Owen 'Anatomy of Vertebrates,' vol. iii. 1868, p. 687. Prof. Turner in 'Edinburgh Medical Journal,' Feb. 1865. |