250 
V. M. BALFOUR. 
])rocess replaces the former process when the hypoblast cells 
are so bulky from being distended by food-yolk that their 
invagination is mechanically impossible. 
e_p 
Fig. 3. — Transverse Section through the Ovum of Euaxes during an 
early stage of development to show the nature of epiholic invagination. 
(After Kowalevsky.) ep. epiblast ; ms. mesoblastic band ; hy. hypo- 
blast. 
There are various peculiar modifications of invagination 
which cannot be dealt with in detail. 
Invagination in one form or other occurs in some or all 
the members of the following groups : 
The Dicyemidae, Calcispongiae (after the amphiblastula 
stage) and Silicispongiae, Coelenterata, Turbellaria, Nemertea, 
ilotifera, Mollusca, Polyzoa, Brachiopoda, Chaetopoda, Dis- 
cophora, Gephyrea, Chaetognatha, Nematelminthes, Crus- 
tacea, Echinodermata, and Chordata. 
The gastrula of the Crustacea is peculiar, as is also that 
of many of the Chordata (Reptilia, Aves, Mammalia), but 
there is every reason to suppose that the gastrulae of these 
groups are simply modifications of the normal type. 
Delamination. — Three types of delamination may 
be distinguished : 
a. Delamination where the cells of a solid morula become 
divided into a superficial epiblast, and a central solid mass 
in which the digestive cavity is subsequently hollowed 
out (fig. 4). 
h. Delamination where the segmented ovum has the 
form of a blastosphere, the cells of which give rise by 
budding to scattered cells in the interior of the vesicle, 
which, though they may at first form a solid mass, finally 
arrange themselves in the form of a definite layer around 
a central digestive cavity (fig. 5), or, in the case of some 
sponges, around several cavities. 
c. Delamination where the segmented ovum has the lorrri 
