102 WILLIAM BATESON. 



have acquired from the ancestor common to them and the 

 Enteropneusta. 



In this way the connection of the Protovertebrata of Balfour 

 with the other division becomes explicable on the new facts 

 derived from the Enteropneusta. 



The peculiar fact that so many of the features of the 

 Enteropneusta differ from those of the Cephalochorda in 

 degree of expression only is very remarkable, and suggest 

 that their further evolution towards the Protochordate type 

 proceeded by correlated variations affecting the several 

 systems. 



From the Protovertebrata thus constituted, which in all 

 probability possessed an unsegmented mesoblastic sheath for 

 the notochord and a brain, the Cyclostomata may be easily 

 derived without the necessity of any hypothesis of great 

 degeneration, which cannot be well supported. 



Balfour has fully discussed the question of the origin of his 

 hypothetical group of Protognathostomata, and upon the 

 question of their immediate origin no new light can be thrown. 



The above suggestions entail many difficulties. The chief of 

 these is that they involve the hypothesis that the rudiment 

 of the notochord of the Archichordata developed itself 

 as a separate structure, once in the case of the Ascidians, and 

 again in the case of the Protochordata. In the first case, 

 owing to the atrophy of the praeoral lobe and use of the tail in 

 swimming, it came to lie in that organ, and in the second case 

 extended through the whole length of the body. Also does this 

 suggestion of the origin of the Tunicates involve the proposition 

 that the rudiment of the dorsal nerve-cord extended itself 

 twice along the body, once in the case of the Ascidians, and 

 again in the case of the Protochordata. If this occurred there 

 is no difficulty in supposing it to have been twice invaginated, 

 this being a more less common feature among nervous 

 systems. 



Another difficulty which affects all these suggestions arises 

 from the epiblastic origin of the generative organs of Enterop- 

 neusta, in which they resemble the Echinoderms. 



