GASTRULATION. 51 
blastula in several ways. The four most important are as 
follows :— 
(1) Archiblastic Invagination._-This method is common in 
the typical (or free-swimming) larva. It consists 
of the tucking-in of the whole of one hemisphere of 
cells, very much as a hollow india-rubber ball when 
punctured may be tucked in. The rim of the hemi- 
sphere gradually narrows to form the blastopore.* 
(2) Unipolar Ingresston.—Single cells at one spot or 
pole of the blastula break away from the archiblast 
and migrate inwards, arranging themselves later as 
an inner layer, the pole of ingression afterwards 
forming the blastopore. 
(3) Multipolar Ingression.—Single cells at indefinite parts 
of the whole archiblast break away and migrate in- 
wards, arranging themselves as an inner layer, a 
blastopore being acquired later as a perforation. 
(4) Delamination.—Each archiblast cell divides into two 
by tangential division and thus the one layer is 
converted into two. A blastopore is then formed 
as a perforation. 
It is probable that multipolar ingression is the most 
primitive of these methods of gastrula production and that 
it leads, on the one hand, to the very ccenogenetic (or 
embryonic) delamination, and, on the other, to unipolar 
ingression and finally invagination. 
Fig. 19. 4. THE PLanuta.—The planula 
SECTION Or PLANULA. bears much the same relation to the 
gastrula as does the morula to the 
blastula. It is an oval larva, formed 
by an outer layer of ciliated epi- 
blastic cells, containing a solid mass 
of hypoblast in its interior. It is 
usually active, free-swimming, and 
marine. It is found very commonly in Calenterata and is 
a coenogenetic modification. 
Hypoblast 
Epiblast 
* In embryonic developments with much yolk the epiblast cells may 
grow gradually over the hypoblast cells, as the latter are too large to be 
tucked into the former. This type of gastrz/atzon (formation of gastrula) 
is termed efzbolic in contrast to the true invagination, often called embolic. 
