336 



DELAMINATION. 



process the inner parts of the cells become separated from the 

 outer, and the walls of the blastosphere are so divided into two 

 distinct layers (fig. 205). 



Although the third of these processes is usually regarded 

 as the type of delamination, it does not, so far as I know, occur 

 in nature, but is most nearly approached in Geryonia (fig. 203). 



The first type of delamination is found in the Ceratospongiae, 

 some Silicispongiae (?), and in many Hydrozoa and Actinozoa, 

 and in Nemertea and Nematelminthes (Gordioidea ?}. The 

 second type occurs in many Porifera \Calcispongics (Ascetta), 

 Myxospongice\ and in some Ccelenterata, and Brachiopoda 

 ( Thecidiiim). 



Delamination and invagination are undoubtedly the two 

 most frequent modes in which the layers are differentiated, but 



C 



FIG. 202. THREE LARVAL STAGES OF EUCOPE POLYSTYLA. (After Kowalevsky.) 

 A. Blastosphere stage with hypoblast spheres becoming budded off into central 

 cavity. B. Planula stage with solid hypoblast. C. Planula stage with a gastric 

 cavity, ep. epiblast ; hy. hypoblast ; al. gastric cavity. 



there are in addition several others. In the first place the 

 whole of the Tracheata (with the apparent exception of the 

 Scorpion) develop, so far as is known, on a plan peculiar to 

 them, which approaches delamination. This consists in the 

 appearance of a superficial layer of cells enclosing a central 

 yolk mass, which corresponds to the hypoblast (figs. 204 and 

 214). This mode of development might be classed under 

 delamination, were it not for the fact that the early development 



