SEGMENT A TION. ^g 



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. ^Bggs with central 

 yolk are often called centro-lecithal^ and those with polar 

 yolk are often called telo-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 stipe^^ficial (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 c^W^d partial segfnentation (see Chick). 



There are numerous transitions and modifications of 

 these types. 



Summary : — 



1. Equal segmentation (retaining centro-symmetry). 



(i) Total 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 Qgg differing from the other). 



( 1 ) Total Unequal. — Found in eggs with moderate 



quantity of yolk, aggregated at one pole. 



(2) Partial Unequal. — Found in eggs with a great 



quantity of yolk. 



Types of Larvae. — Several important larval types are 

 found in the Metazoa. 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 : — 



I. Monoblastic Larvae. 



I. The Blastula. — 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 



