124 BULLETIN OF THE 
shown by Metschnikoff (86, pp. 66-68, Taf. X.), there is a typical in- 
vagination. 
If the observations of McMurrich (’91, p. 314) on Cyanea arctica are 
substantiated, we have among the Scyphomeduse one example of the 
formation of a sterrula by ingression, with the subsequent formation of a 
gastrula-like structure, without an invagination. From the preceding 
summary it is to be seen that there are in Scyphomedus® two cases in 
which the mode of gastrulation appears to be intermediate between 
ingression and invagination, and at least four cases of unquestionable 
invagination. If, in the light of so much variation in the mode of 
gastrulation in this group as is shown by the few forms studied, it is 
safe to conclude that any one mode is typical, that mode would cer- 
tainly appear to be invagination, and not, as Hamann and McMurrich 
have recently maintained, ingression. 
CAMBRIDGE, June 20, 1891. 
