THE RECAPITULATION THEORY 349 



primitive form of the liver is that of Amphioxus, in 

 which it is present as a simple saccular diverti- 

 culum of the intestinal canal, with its wall consisting 

 of a single layer of cells, and with bloodvessels on 

 its outer surface. The earliest stage in the forma- 

 tion of the liver in higher vertebrates the frog, 

 for instance is practically identical with this. In 

 the frog the next stage consists in folding of the 

 wall of the sac, which increases the efficiency of 

 the organ by increasing the extent of surface in 

 contact with bloodvessels. The adult condition is 

 attained simply by a continuance of this process ; 

 the foldings of the wall becoming more and more 

 complicated, but the essential structure remaining 

 the same a single layer of epithelial cells in con- 

 tact on one side with bloodvessels, and bounding 

 on the other directly or indirectly the cavity of the 

 alimentary canal. 



It is not always possible to point out the 

 particular advantage gained at each step even 

 when a complete developmental series is known 

 to us, but in such cases, as for instance in 

 Orbitolites, our difficulties arise chiefly from ignor- 

 ance of the particular conditions that confer 

 advantage in the struggle for existence in the case 

 of the forms we are dealing with. 



The early larval stages in the development of 

 animals, and more especially those that are marine 

 and pelagic in habit, have naturally attracted much 

 attention, since in the absence, probably inevitable, 

 of satisfactory palaeontological evidence, they afford 

 us the sole available clue to the determination of the 



