THE DKVELOPMIONT OF LEPIDOSIREN PARADOXA. 33 



Figs. 101 and 102 in Hert\vig-'s ' Lelirbuch '^ are sufficient 

 to illustrate Ins view of the derivation of the mesodermic 

 rudiments in the higher vertebrates from the enterocoelic 

 pouches of Amphioxus. This view, as is well known, 

 rests mainly on Hertwig's observations on the development 

 of Triton, in which he found pouches of the archenteric 

 cavity projecting on each side of the notochord into the 

 mesoblastic rudiment, which pouches he interpreted as 

 vestiges of the original communications between the archen- 

 teron and the cavity of enterocoelic mesoderm poaches like 

 those of Amphioxus. These observations of Hertwig 

 appear to have failed to find adequate confirmation, and it 

 seems to me that a scheme such as that represented below 

 fits in better with the general facts of vertebrate develop- 

 ment. Such a scheme, as will be seen, agrees in general 

 principle with the theory suggested by Lankester and de- 

 veloped especially by 0. Hertwig, that the mesodermal 

 rudiments on each side of the vertebrate embryo represent 

 the walls of the enterocoelic pouches of Amphioxus; it 

 differs from the Hertwig development of the theory in the 

 detail that it regards the continuity often found in verte- 

 brate embrj'^os between mesoderm and hypoblast on each side 

 of the notochord (and necessarily also the similar continuity 

 between mesoderm rudiment and notochord) as being a 

 secondary fusion rather than as representing the original 

 connection of mesodermic diverticulum with wall of the 

 archenteron. 



The adjoining figures (Text-fig. 5) represent transverse 

 sections through the embryos of Amphioxus, Petromyzon, 

 Lepidosireu, and chick. In the case of the last three I 

 have, for convenience, represented only a small portion of 

 the whole section. As will be seen, the condition in Petro- 

 myzon agrees very closely with that in Amphioxus, and 

 is immediately derivable from it by reduction in the size of 

 the archenteric cavity. 



The disappearance of the cavity of the enteric diverticulum 



' 6" Auflage. 



VOL,. 45, TAUr 1. NKW SERIES. C 



