THE DEVIiLOPMENT OF LEPIDOSIREN FARADOXA. 31 



Formation of Secondary Enteric Roof. — Brauer lias 

 described in Gymnophiona the formation of the definitive 

 enteric roof by a backgrowth of ^S'egetative cells" under the 

 original archenteric roof. In Lepidosiren no such back- 

 growth takes place. It is to be noted, however, that there is 

 much variation in the character of the yolk gi-anules in the 

 cells lining the archenteric roof immediately ventral to the 

 chorda. Most usually these cells have fine granules, but very 

 frequently, on the other hand, they are distinctly marked oif 

 from the chorda cells by their yolk remaining in much coarser 

 granules (cf. PI. 4, fig. 17). With only scanty material, in 

 which the later stages happened to show this difi'orence, one 

 might well imagine it due to a secondary growth of large 

 yolked cells in beneath the chorda rudiment. In view of 

 this possibility of erroneous interpretation of sections I do 

 not feel absolutely convinced that such a backwai-d growth 

 of large yolk-cells under the original cells of the archenteric 

 roof as has been described by Brauer and also by Lwoif 

 actually takes place. 



In regard to Brauer's observations I might add that in 

 my personal opinion the large-grained character of the cells 

 figured by him as growing backwards makes it unlikely that 

 they are multiplying with the activity which would be 

 necessary on his view. 



In regard to Brauer's fig. 59,^ where the large yolked cells 

 extend right to the blastopore, it is of importance to note that 

 the author expressly states that it is not a median section. 

 In Lepidosiren it is only the median part of the archen- 

 teric roof that is fine-grained. 



In Ccratodus Semon has described the roof of the 

 enteron as being formed by an ingrowth from each side 

 under the chorda rudiment. There is no such ingrowth in 

 Lepidosiren. Where it does occur it may be looked on 

 as a CEenogenetic modification bearing the same relation to 

 the method of chorda formation in Amphioxus, as the 

 method of separation of the neural rudiment from the 



' ' Zool. Jalub. Aiiat.,' Bii. x, Taf. xxxvii. 



