836 REPORT—1890. 
stage in the pedigree. The early attainment of the cyclical form in the shell of 
Orbitolites complanata is a case in point; and Wiirtenberger has specially noticed 
this tendency in Ammonites. Many early larve show it markedly, the explana- 
tion in this case being that it is essential for them to hatch in a condition capable 
of independent existence, #.e., capable, at any rate, of obtaining and digesting their 
own food. 
Anachronisms, or actual reversal of the historical order of development of organs 
or parts, occur frequently. Thus the joint surfaces of bones acquire their charac- 
teristic curvatures before movement of one part on another is effected, and before 
even the joint cavities are formed. 
Another good example is afforded by the development of the mesenterial 
filaments in Alcyonarians. Wilson has shown in the case of Renilla that in the 
development of an embryo from the egg the six endodermal filaments appear first, 
and the two long ectodermal filaments at a later period; but that in the formation 
of a bud this order of development is reversed, the ectodermal filaments being the 
first formed. He suggests, in explanation, that as the endodermal filaments are 
the digestive organs, it is of primary importance to the free embryo that they 
should be formed quickly. The long ectodermal filaments are chiefly concerned 
with maintaining currents of water through the colony ; in bud-development they 
appear before the endodermal filaments, because they enable the bud during its 
early stages to draw nutrient matter from the body fluid of the parent; while the 
endodermal filaments cannot come into use until the bud has acquired both mouth 
and tentacles. 
The completion of the ventricular septum in the heart of higher vertebrates 
before the auricular septum is a well-known anachronism, and every embryologist 
could readily furnish many other cases. 
A curious instance is afforded by the development of the teeth in mammals, if 
recent suggestions as to the origin of the milk dentition are confirmed, and the 
milk dentition prove to be a more recent acquisition than the permanent one.! 
But the most important cases in reference to distortion in time concern the 
reproductive organs. If development were a strict and correct recapitulation of 
ancestral history, then each stage would possess reproductive organs in a mature 
condition. This is not the case, and it is clearly of the greatest importance that it 
should not be. It is true that the first commencement of the reproductive organs 
may occur at a very early larval stage, or even that the very first step in develop- 
ment may be a division of the egg into somatic and reproductive cells; and it is 
possible that, as maintained by Weismann, this latter condition is a primitive one. 
Still, even in these cases the reproductive organs merely commence their develop- 
ment at these early stages, and do not become functional until the animal is 
adult. 
Exceptionally in certain animals, and as a normal occurrence in others, 
precocious maturation of the reproductive organs takes place, and a larval form 
becomes capable of sexual reproduction. This may lead to arrest of development, 
either at a late larval period as in the Axolotl, or at successively earlier and earlier 
stages, as in the gonophores of the Hydromeduse, until finally the extreme condi- 
tion seen in Hydra is produced. 
We do not know the causes that determine the period, whether late or early, 
at which the reproductive organs ripen, but the question is one of great interest 
and importance and deserves careful attention. The suggestion has been made 
that entire groups of animals, such as the Mesozoa, are merely larve, arrested 
through such precocious acquiring of reproductive power, and it is conceivable 
that this may be the case. Mesozoa are a puzzling group in which the life history, 
though known with tolerable completeness, has as yet given us no reliable clue 
concerning their affinities to other animals, a tantalising distinction that is shared 
with them by Rotifers and Polyzoa. 
1 Cf. Thomas Oldfield, ‘On the Homologies and Succession of the Teeth in the 
Dasyuride, with an attempt to trace the history of the evolution of the Mammalian 
teeth in general,’ Phil. Trans. 1887. 
