NA TURE 



505 



THURSDAY, MARCH 31, 1898. 



C.^NOGENESIS, THE EXPRESSION OF 

 VARIOUS PHYLOGENETIC ENERGIES. 

 Kainogenesis ah Ausdruck differenter phylogenetischer 

 Energien. Von Dr. Ernst Mehnert, Privat-docent an 

 der Universitiit Strassburg i. Elsass. Pp. 165, and 

 plates. (Jena : Gustav Fischer, 1897.) 



IT has become more and more evident, during the 

 past twenty years, that the course of ontogeny may 

 vary in a quite remarkable degree, even within the limits 

 of a genus, and that the individuals of a species may 

 present not unimportant differences in their respective 

 ontogenies. This contradiction of the old-established 

 belief that the type of development was similar for every 

 considerable group of animals, has led to the abandon- 

 ment of many of the older principles of embryology, and 

 has introduced no small amount of confusion into em- 

 bryological speculations. The question which confronts 

 the investigator is this : which among the several modes 

 of development which occur in closely allied species is to 

 be regarded as primitive, and which secondary ? In 

 other words, what developmental features are palingenetic 

 and what caenogenetic ? 



In the present work Dr. Mehnert seeks an answer 

 to the question by a comparison of the developmental 

 characters exhibited in homologous organs in individual 

 embryos and in individual species with the characters 

 exhibited by the same organs in adult individuals. 



The definitive condition of an organ in an adult indi- 

 vidual is, he says, the direct product of its phylogeny : it 

 is therefore possible, by means of such a comparison, to 

 recognise the characteristic correlative differences which 

 obtain between the several ontogenies of homodynamous 

 organs and their different degrees of phyletic perfection. 

 He proposes to establish the principle that the law of 

 close interdependence between phylogeny and ontogeny 

 leads, as a consequence, to the appearance of caeno- 

 genesis ; in other words, that caenogenesis is not, as is 

 often assumed, an anomalous phenomenon, but is 

 governed by strictly regulated principles. 



In order to establish his thesis. Dr. Mehnert gives the 

 results of his extensive researches on the fore and hind 

 limbs of pentadactyle Vertebrates. He selected these 

 as most appropriate to his purpose because the hand and 

 foot are serially homologous, and because each is an 

 organ composed of several dozen separate but approxi- 

 mate and partly homodynamous skeletal pieces, whose 

 homologies can with some certainty be asserted in the 

 whole group of pentadactyle Vertebrates. The different 

 stages of prechondrified, chondrified, and ossified tissues 

 also afford a series of changes whose estimation presents 

 no difficulty. 



Although he has worked over a great number of 

 species. Dr. Mehnert confines himself to the detailed 

 description of a few forms, selecting Emys lutaria, 

 Struthio, some carinate birds, some Mammals, and some 

 Amphibia. 



The hand of Emys is, as is well known, typical. 

 There are five digits, a distal row of five carpals, a 

 proximal row of three carpals (radiale, intermedium and 

 NO. 1483, VOL. 57] 



ulnare) and a centrale. In addition, Dr. Mehnert recog- 

 nises certain " sesamoid " bones, an ulnare externum, a 

 radiale externum, a centrale ulnare, and a centrale dis- 

 tale as belonging to a regressive skeletal series which 

 was ancestrally equivalent to the other carpalia. 

 He finds that the pendactyle arrangement is early 

 sketched out in prechondrial tissue, and that chondrifica- 

 tion begins in the forearm, and proceeds distally, each 

 transverse row of carpalia, metacarpalia and phalanges 

 chondrifying at the same time, the different rows succes- 

 sively in centrifugal order. This condition is compared 

 to that of the foot of Struthio, which in the adult is two- 

 toed, the third and fourth digits alone being developed. 

 In young embryos, however, the foot is discovered to be 

 pentadactyle ; only three toes are visible externally, but 

 in sections all five may be distinguished. The centres of 

 chondrification appear successively, beginning in the 

 proximal elements, and extending at short intervals of 

 time to the distal elements. Of the metatarsalia, the 

 third and fourth, corresponding to the permanent toes of 

 the adult, are the first to be chondrified, the second and 

 fifth are chondrified later ; the first, being very rudimen- 

 tary, disappears early. The conclusion is that in develop- 

 ment permanent structures are accelerated, evanescent 

 structures retarded. The same principle is illustrated 

 by the development of the wing, and even more remark- 

 ably, for whereas it is usual in Vertebrata that the hind 

 limb should lag behind the fore limb in development, in 

 Struthio, and also in Apteryx, the adult reduction of the 

 fore limb is foreshadowed in the embryo, since it lags 

 behind the hind limb from the first. The same phen- 

 omenon is observed in Anura, in the adults of which the 

 hind limbs are predominant as compared with the fore 

 limbs. From a number of considerations, such as the 

 limb of Baptanodon, and the embryonic condition of the 

 limbs of Emys, Mehnert concludes that all the skeletal 

 elements of the limb were primitively of equal size, and 

 somewhat short and square in shape. If this were so, the 

 long bones and the metacarpals, metatarsals and phalanges 

 would be progressive elements, which have increased in 

 length in the course of phylogeny, whilst the carpals or 

 tarsals would be regressive elements, and on his principle 

 the last-named should show evidence of retarded, the 

 former of accelerated development. This he shows to be 

 the case in Mammals and in carinate birds, in which the 

 metacarpal precedes the carpal differentiation. By the 

 comparison of a large number of forms Mehnert arrives 

 at the general rule, that all the permanent digital rays 

 are histologically differentiated at a very early period, 

 and are the first to be divided into their components : 

 in the regressive lateral digits the reverse is the case — 

 they lag far behind the predominant digits, and they may 

 eventually degenerate and disappear. 



Mehnert claims to have shown that the ontogenetic 

 retardation of the development of an organ is connected 

 with a phyletic diminution of its functional importance. 

 This he has undoubtedly done, and the special part of 

 his work is full of interesting facts bearing upon minor 

 anatomical problems. It is not, however, easy to follow 

 him in his excursion into theoretical questions. Having 

 asserted the nature of the parallelism between ontogeny 

 and phylogeny, he proceeds to seek for a causal explan- 

 ation, and finds it in the diminution and final loss of the 



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