124 BULLETIN OF THE 



shown by Metsclmikofli' ('86, pp. 66-G8, Tuf. X.), there is a typical in- 

 vagination. 



If the observations of McMurrich ('91, p. 314) on Cyanea arctica are 

 substantiated, we have among the Scyphomedusse 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 Scyphomedusaj 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. 



