278 



JJKLAMI NATION. 



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 [Calcispongiw {Ascetta), Myxospongicje], and 

 in some Coelenterata, and Brachiopoda {Thecidiimi). 



Delamination and invagination are undoubtedly the two most 



Fig. 202. Three L.iRVAL, stages of Eucope polystyla. (After Kowalevsky.) 

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



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



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



frequent modes in which the layers are differentiated, but 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 delami- 

 nation. This consists in the appearance of a superficial layer of 

 cells enclosing a central yolk mass, which corresponds to the hypo- 

 blast (figs. 204 and 214). This mode of development might be 

 classed under delamination, were it not for the fact that the early 

 development of many Crustacea is almost the same, but is subsequently 

 followed by an invagination (fig. 208), which apparently corresponds 

 to the normal invagination of other types. There are strong grounds for 

 thinking that the tracheate type of formation of the epiblast and 

 hypoblast is a secondary moiMfication of an invaginate type {vide 

 Vol. I. p. 378). 



The type of some Turbellaria (Stylochopsis ponticus) and that of 

 Nephelis amongst the Discophora is not capable of being reduced to 

 the invaginate type. 



