SEGMENTATION. 49 
of its presence would probably be a retardation of seg- 
mentation, but it is usually either aggregated towards the 
centre of the egg or in one hemisphere. Eggs with central 
yolk are often called centro-dectthal, and those with polar 
yolk are often called ¢e/o-lecithal, 
In centro-lecithal eggs the segmentation is usually equal, 
but the presence of the yolk retards or prevents the inner 
part from segmenting; hence this type of segmentation is 
called superficial (see Nephrops). In telo-lecithal eggs, if 
the yolk be not too great in amount, it merely retards the 
segmentation of the hemisphere in which it is situated and 
we have a ‘total unequal segmentation (see Frog). In 
many telo-lecithal eggs, however, the amount of yolk is so 
enormous that it entirely prevents segmentation of the 
part occupied by it and the cell-formation only proceeds at 
one pole. This is called partial segmentation (see Chick). 
There are numerous transitions and modifications of 
these types. 
SUMMARY :— 
1. Equal segmentation (retaining centro-symmetry). 
(1) Yotal Equal.— Found in eggs with no yolk (alecithal) 
or with evenly distributed yolk. 
(2) Partial Equal (superficial).—Found in eggs with yolk 
aggregated symmetrically round the centre. 
2. Unequal segmentation (showing axo-symmetry, one 
pole of the egg differing from the other). 
(1) Total Unequal.—Found in eggs with moderate 
quantity of yolk, aggregated at one pole. 
(2) Partial Unegual.— Found in eggs with a great 
quantity of yolk. 
Types of Larvze.—Several important larval types are 
found in the Jefazoa. Many of them occur in several 
groups and with sufficient persistency to indicate that 
they represent phyletic stages. We may briefly note some 
of the following :— 
1. Monoblastic Larve. 
1, THE BiastuLA.—The blastula larva is a hollow ball 
of cells of one-cell thickness, it is usually free-swimming 
and marine, and the cells bear either cilia or flagella, 
M. 5 
