NOTES ON EMBRYOLOGY AND CLASSIFICATION. 409 



ever, the observations already cited concerning the Geryonidse 

 and some observations of Kowalewsky on Alcyonium, and 

 on species of Actiniae, which indicate the development of an 

 enteric cell-layer by Delamination. In the development of 

 the enteron by Invagination, usually a number of cells become 

 depressed in a cup-like manner on the surface of the blastula, 

 sinking more and more deeply into the blastocoel (if such 

 cavity be present), until the once spherical blastula (Fig. 11) 

 has become a hemispherical cup, built up of two cell-layers, 

 one pressed against the other (Fig. 15). The rim of the cup 

 now contracts, and frequently closes ; the wider or narrower 

 orifice thus formed I have termed the blastopore. The case just 

 described is that of an organism in which the egg-cell contains 

 relatively a small amount of food- material, and when conse- 

 quently the cells of the morula and of the blastula are of 

 nearly equal size. Haeckel has called this the Archiblastic 

 type. When there is more food-material in the egg, it 

 either collects to the centre of the mass of cells as division 

 proceeds, and segregates in a way which strongly suggests 

 delamination,! lying eventually in the central cavity of a 

 vesicle (blastula), formed by a single layer of cells (Periblas- 

 tic type, Haeckel), or the food-material is associated from the 

 first post-seminate phases of the egg-cell with one hemisphere 

 or larger moiety of the egg, namely, that which is destined 

 to form the enteric cell-layer, whilst the part of the e^g 

 (often extremely small) which is destined to form the deron 

 or ectoderm is free (or becomes so by segregation) from such 

 food-material. The consequence of this arrangement is that 

 the enteric moiety of the egg-cell is separated in the earlier 

 phases of cleavage from the deric moiety (Fig. 12), and is 

 not only more bulky, but breaks up into new cells more 

 slowly than the latter, so that it becomes overgrown by the 

 deric cells rather than invaginated into them (Fig. 13). 

 Two degrees of this Epibolic mode of Invagination (so called 

 by Selenka, in distinction from the Embolic mode) are dis- 

 tinguished by Haeckel as Amphiblastic and Discoblastic. 



It is clear enough that the special modifications of the 

 process of invagination due to the presence in the egg of a 

 large amount of food-material may be dismissed in consider- 

 ing the question as to how the process of Delamination came 

 to be replaced by that of Invagination, since the presence of 

 such an excess of food-material is a secondary and late con- 

 dition. The facts, however, connected with the behaviour 

 of the food-material, when present, suggest the explanation 



' The formation of the enteron in periblastic forms such as the Arthro- 

 poda and Anthozoa (Alcyouium) requires much further study. 



