BLASTULA AND GASTRULA. 67 
[or, Ova with a considerable amount of deutoplasm lying towards. 
one ‘ag (telolecithal), undergo unequal holoblastic segmenta- 
tion]. 
This is illustrated in some Sponges, some Ccelentera (e.g. 
Ctenophora), some ‘‘ Worms,” many Molluscs, the lamp- 
rey, Ganoid Fishes, Dipnoi, Amphibians (Fig. 31 (2)). 
B, ParTiaL DivistoN—Meroblastic Segmentation 
(3) Eggs with a large quantity of yolk on which the formative 
protoplasm lies as a small disc at one pole, divide partially, 
and in discoidal fashion, 
[or, Ova which are telolecithal, and have a large quantity of 
deutoplasm, undergo merobiastic and discoidal segmentation]. 
This is illustrated in all Cuttle-fishes, all Elasmobranch and 
Teleostean Fishes, all Reptiles and Birds (Fig. 31 (3)), 
and also in the Monotremes or lowest Mammals. 
(4) Eggs with a considerable quantity of yolk accumulated in a 
central core and surrounded by the formative protoplasm,,. 
divide partially, and superficially or peripherally, 
{or, Ova which are centrolecithal undergo meroblastic and super- 
ficial segmentation]. 
This is illustrated by most Arthropods (Fig. 31 (4)), and’ 
by them alone. 
Blastosphere and morula.—The result of the division is. 
usually a ball of cells. But when the yolk is very abundant 
(3), a disc of cells—a discoidal blastoderm—is formed at 
one pole of the mass of nutritive material, which it gradually 
surrounds. 
As the cells divide and redivide, they often leave a large 
central cavity—the segmentation cavity—and a hollow ball 
of cells—a blastosphere or blastula—results. 
But if the so-called ‘segmentation cavity” be very small 
or absent,.a solid ball of cells or morula, like the fruit of 
bramble or mulberry, results. 
Gastrula.—The next great step in development is the 
establishment of the two primary germinal layers, the outer 
ectoderm and the inner endoderm, or the epiblast and the 
hypoblast. 
One hemisphere of the hollow ball of cells may be appar- 
ently dimpled into the other, as we might dimple an india- 
rubber ball which had a hole init. Thus out of a hollow 
ball of cells, a two-layered sac is formed—a gastrula formed 
by invagination or emébolé (Fig. 32). The mouth of the 
gastrula is called the blastopore, its cavity the archenteron. 
