42 PROFESSOR A. M. MARSHALL. 



The latter explanation is the one usually adopted ; but before the 

 question can be finally decided more accurate observations than we at 

 present possess are needed concerning the stages intermediate between 

 the egg and the Nauplius. 



The absence of a heart in the Nauplius may reasonably be associated 

 with the small size of the larva. 



Concerning the larval forms of vertebrates, it is only in Amphioxus 

 and the 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 customary, 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. 



