GASTRULATION. 51 



blastula in several ways. The four most important are as 



follows : — , 



( I ) 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 Ingression. — 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) Delaminaiion. — 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 coenogenetic (or 



embryonic) delaminaiion, and, on the other, to unipolar 



ingression and finally invagination. 



F^g- ^9- 4. The Planula.— The planula 



Section of Planula. bg^^s much the same relation to the 



gastrula as does the morula to the 



Hypoblast ,^^^^ blastula. It is an oval larva, formed 



^S^^^Q by an outer layer of ciliated epi- 



Epibiast l^^^^fl 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 Coelenterata and is 



a coenogenetic modification. 



* 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 oi gastrulation (formation of gastrula) 

 is termed epibolic in contrast to the true invagination, often called embolic. 



