l, and he held that the ancestral stages
distinguished by Oppel could not be satisfactorily established. He
suggested an interesting explanation of heterochrony in development,
according to which the premature or retarded appearance of organs in
ontogeny stands in close relation with the time of their entering upon
functional activity. Thus in many mammals the mesodermal part of the
allantois often appears long before the endodermal part, though this is
phylogenetically older. This Keibel ascribes to the fact that the
endodermal part is almost functionless. "One can directly affirm," he
writes, "that the time of appearance of an organ depends in an eminent
degree upon the time when it has to enter upon functional activity. This
moment is naturally dependent upon the external conditions. Among the
highest Vertebrates, the mammals, the traces of phylogeny shown in
ontogeny are to a great extent obliterated through the adaptation of
ontogeny to the external conditions, and through the modifications which
the germs of more highly organised animals necessarily exhibit from the
very beginning as compared with germs which do not reach such a high
level of development" (p. 754, 1897).

Study of individual variation in the time of appearance of the organs in
embryos of the same species was prosecuted with interesting results by
Bonnet,[531] Mehnert,[532] and Fischel.[533] Fischel found that variability
was greatest among the younger embryos, and became progressively less in
later stages. Like von Baer (_supra_, p. 114) he inferred that
regulatory processes were at work during development which brought
divergent organs back to the normal and enabled them to play their part
as correlated members of a functional whole.

Important theoretical views were developed by Mehnert[534] in a series of
publications appearing from 1891 to 1898. Like Keibel, Mehnert
emphasised the importance of function in determining the late or early
appearance of organs, but he conceived the influence of function to be
exerted not only in ontogeny, but also throughout the whole course of
phylogeny, by reason of the transmission to descendants of the effects
of functioning in the individual life.

In his paper of 1897 Mehnert details the results of an extensive
examination of the development of the extremities throughout the Amniote
series. He finds that in all cases a pentadactylate rudiment is formed,
even in those forms in which only a few of the elemen