TRANSACTIONS OF SECTION D. 849 
Ascidians that the earliest larval stages are free-living, independent animals. In 
both groups the most characteristic larval stage is that in which a notochord is 
present, and a neural tube, open in front, and communicating behind through a 
neurenteric canal with the digestive cavity, which has no other opening to the 
exterior. This is a very early stage, both in Amphioxus and Ascidians; but, so 
far as we know, it cannot be compared with any invertebrate larva. It is custo- 
mary, in discussions on the affinities of vertebrates, to absolutely ignore the 
vertebrate larval forms, and to assume that their peculiarities are due to precocious 
development of vertebrate characteristics. It may turn out that this view of the 
matter is correct; but it has certainly not yet been proved to be so, and the 
development of both Amphioxus and Ascidians is so direct and straightforward 
that evidence of some kind may reasonably be required before accepting the 
doctrine that this development is entirely deceptive with regard to the ancestry of 
vertebrates. 
Zoologists have not quite made up their minds what to do with Amphioxus : 
apparently the most guileless of creatures, many view it with the utmost suspicion, 
and not merely refuse to accept its mute protestations of innocence, but regard and 
speak of it as the most artful of deceivers. Few questions at the present day are 
in greater need of authoritative settlement. 
That ontogeny really is a repetition of phylogeny must, I think, be admitted, 
in spite of the numerous and various ways in which the ancestral history may be 
distorted during actual development. 
Before leaving the subject, it is worth while inquiring whether any explanation 
can be found of recapitulation. A complete answer can certainly not be given at 
present, but a partial one may, perhaps, be obtained. 
Darwin himself suggested that the clue might be found in the consideration 
that at whatever age a variation first appears in the parent, it tends to reappear at 
@ corresponding age in the offspring ; but this must be regarded rather as a state- 
ment of the fundamental fact of embryology than as an explanation of it. 
It is probably safe to assume that animals would not recapitulate unless they 
were compelled to do so: that there must be some constraining influence at work, 
_ forcing them to repeat more or less closely the ancestral stages. It is impossible for 
instance to conceive what advantage it can be to a reptilian or mammalian embryo 
_ to develop gill clefts which are never used, and which disappear at a slightly later 
stage; or how it can benefit a whale, that in its embryonic condition it should 
possess teeth which never cut the gum, and which are lost before birth. 
Moreover, the history of development in different animals or groups of animals, 
offers tc us, as we have seen, a series of ingenious, determined, varied, but more or 
less unsuccessful efforts to escape from the necessity of recapitulating, and to 
_ Substitute for the ancestral process a more direct method. 
A further consideration of importance is that recapitulation is not seen in all 
forms of development, but only in sexual development ; or, ut least, only in 
development from the ege. In the several forms of asexual development, of which 
budding is the most frequent and most familiar, there is no repetition of ancestral 
_ phases ; neither is there in cases of regeneration of lost parts, such as the tentacle 
_of a snail, the arm of a starfish, or the tail of a lizard; in such regeneration it is 
not a larval tentacle, or arm, or tail, that is produced, but an adult one. 
The most striking point about the development of the higher animals is that 
they all alike commence as eggs. Looking more closely at the egg and the condi- 
tions of its development, two facts impress us as of special importance : first, the 
egg is a single cell, and therefore represents morphologically the Protozoan, or 
earliest ancestral phase ; secondly, the egg, before it can develop, must be fertilised 
by aspermatozoon, just as the stimulus of fertilisation by the pollen grain is neces- 
sary before the ovum of a plant will commence to develop into the plant-embryo. 
The advantage of cross-fertilisation in increasing the vigour of the offspring is 
well known, and in plants devices of the most varied and even extraordinary kind 
_are adopted to ensure that such cross-fertilisation occurs. The essence of the act of 
_cross-fertilisation, which is already established among Protozoa, consists in com- 
