252 
F. M. BALFOUR. 
the outer, and the walls of the blastosphere are so divided 
into two distinct layers (fig. -8). 
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. 6). 
The first type of delamination is found in the Cerato- 
spongise, some Silicispongiae (?), and in many Hydrozoa and 
Actinozoa, and in Nemertea and Nematelminthes [Gordi^ 
oidea?). The second type occurs in many Porifera [^Calci- 
spongi(B (^Ascetta), Myxospongi< 2 ~\y and in some Coelenterata, 
and Brachiopoda [Thecidium) . 
Tig. 6 . — Diagramatic Figures showing the Felamination of the Fmhryo 
of Geryonia. (After Eol.) A. Stage at the commencement of the 
delamination ; the dotted lines show the course of the next planes of 
division. B. Stage at the close of the delamination, cs. segmentation 
cavity ; a. endoplasm ; b. ectoplasm. 
Delamination and invagination are undoubtedly the two 
most 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 of their 
own, 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. 7 and 
17). This mode of development might be classed under 
delamination were it not for the fact that the early develop- 
ment of the Crustacea is almost the same, but is subsequently 
followed by an invagination (fig. 11), which apparently 
corresponds to the normal invagination of other types. 
There are grounds for thinking that the tracheate type of 
formation of the epiblast and hypoblast is a secondary 
modification of an invaginate type. 
The type of some Turbellaria {Stijlocopsis ponticus) and 
