236 Mr. E. B. kankester on the [Mar. 12, 



of imagination. There is no question of " epiboly " here, but a direct 

 and simple splitting of one cell into two ; so that what was a sac formed 

 by a layer of cells one deep, becomes a sac formed by a layer of cells 

 two deep, or of two layers each one deep. 



It is yet a question for much further inquiry as to how this mode 

 of forming a double-walled Gastrula can be derived from, or harmonized 

 with, the formation of Gastrulce by the embolic or epibolic forms of inva- 

 gination. 



It would certainly seem at present that the orifice of invagination of 

 the invaginate Gastrula must not be regarded as the equivalent of the 

 later erupting mouth of the segregate Gastrula*, which is the true per- 

 manent mouth of the Sponge or Coalenterate. In no case is the orifice 

 of invagination of the invaginate Gastrula known to persist under any 

 form ; it appears solely to effect the invagination, and when that is 

 effected vanishes. 



Enough has been said to show the importance of observations relating 

 to the 6rosraZa-phase of development. In the paper well-marked inva- 

 ginate Gastrulce are described from : 



1. Pisidium (Lamellibranch). 



2. Tergipes (Nudibranch). 



3. Polycera (Nudibranch). 



4. Limaoc (Pulmonate). 



5. Limnceus (Pulmonate). 



In addition to these cases of the development of invaginate Gastrulce 

 among Mollusca, the examination of the very beautiful figures in the 

 papers of Loven on molluscan development leaves no doubt that he 

 has observed invaginate Gastrulce in the following cases, but has not 

 understood their structure : 



6. Cardium (Lamellibranch). 



7. Crenella (Lamellibranch). 



Similarly, Karl Yogt's observations on Actceon indicate the same state 

 of things as the author has pointed out in Polycera ; and hence we may 

 add:- 



8. Actceon (Nudibranch), 



and, finally, from Kowalevsky's statement, though not accompanied by 

 figure or description, 



9. Atalanta (Heteropod). 



The second matter of theoretical interest (namely, the early features 

 in the development of the shell) has not been previously discussed, since 

 the structures described in the paper as shell-patch, shell-groove, and 

 shell-plug were unknown. 



If, as seems justifiable, the Cephalopoda are to be regarded as more 



* In his paper in the 'Annals' for May 1873 the author has inclined to the view 

 that it may be so regarded. 



