28 J. GRAHAM KERR. 



My own couclusions with regard to tlie part played by back- 

 ward movement of the blastopore lip agree closely with 

 those reached by Kopsch for Amphibia, and my support of 

 his views is strengthened by the fact that I had not seen his 

 paper until I had finished my observations of the phenomenon 

 in L epi do siren. 



As will have been gathered from the descriptive part of this 

 paper, I am strougly of opinion that in Lepidosiren the 

 main factor in the formation of the archenteron is a process 

 of invagination. I am not at present, however, prepared to 

 deny that during what I have called Stage B of gastrula- 

 tion this process may not be aided to some extent by 

 splitting. 



Communication between Archenteron nnd Seg- 

 mentation Cavity. — The view expressed by Kupffer in 

 1879,^ that the enteron is formed by a fusion of the two 

 originally separate cavities — archenteron and segmentation 

 cavity — has recently been supported for the large eggs of 

 Salamandra maculosa and G-y mnophiona. It will be 

 seen from figs. 8 and 11 how thin is the septum separating 

 these cavities, and how easily they might be thrown into one 

 by rupture of the intervening wall. In one or two eggs I 

 have found this happen. I attribute it to the fixing fluid 

 not having penetrated properly ; but whether this be so, or 

 whether it really existed in the living egg, it is in any case 

 quite abnormal in Lepidosiren, and in all except these 

 few exceptional cases the two cavities remain completely 

 shut off. 



Formation of Parts of Archenteric Roof from 

 " Ectodermic '' or "Animal" Cells. — In the preceding- 

 description I have made no statements regarding ingrowth 

 of ectodermic or animal cells along the roof of the arch- 

 enteron. Assertions that this occurs in other forms seem to 

 me to be weakened by two fallacies. In the statement by 

 Lwoff, Brauer, and others that the plate above the arch- 

 enteron which gives rise to chorda and mesoderm is ecto- 

 ' *Zool. Anzeiger,' ii, p. 594. 



