1 10 CENTROLECITHAL SEGMENTATION. 



It is convenient to distinguish the ova which segment 

 uniformly by some term ; and I should propose for this the 

 term alecithal 1 , as implying that they are without food-yolk, 

 or that what little food-yolk there is, is distributed uniformly. 



The ova in which the yolk is especially concentrated at one 

 pole I should propose to call telolecithal. They constitute 

 together a group with an unequal or partial segmentation. 



The telolecithal ova may be defined in the following way : 

 ova in which the food-yolk is not distributed uniformly, but is 

 concentrated at one pole of the ovum. When only a moderate 

 quantity of food-yolk is present the pole at which it is concen- 

 trated merely segments more slowly than the opposite pole ; but 

 when food-yolk is present in very large quantity the part of the 

 ovum in which it is located is incapable of segmentation, and 

 forms a special appendage known as the yolk-sack. 



There is a third group of ova including a series of types of 

 segmentation nearly parallel to the telolecithal group. This 

 group takes its start from the alecithal ovum as do the teloleci- 

 thal ova, and equally with these includes a series of varieties 

 of segmentation running parallel to the regular and unequal 

 types of segmentation which directly result from the presence 

 of a greater or smaller quantity of food-yolk. The food-yolk is 

 however placed, not at one pole, but at the centre of the ovum. 

 This group of ova I propose to name centrolecithal. It is 

 especially characteristic of the Arthropoda, if not entirely con- 

 fined to that group. 



Centrolecithal ova. As might be anticipated on the analogy 

 of the types of segmentation already described, the concentration 

 of the food-yolk at the centre of the ovum does not always take 

 place before segmentation, but is sometimes deferred till even 

 the later stages of this process. 



Examples of a regular segmentation in centrolecithal ova 

 are afforded by Palaemon (Bobretzky) and Penaeus (Haeckel). 

 A type of unequal segmentation like that of the Frog occurs in 

 Gammarus locusta (Beneden and Bessels), where however the 

 formation of a central yolk mass does not appear to take place 



1 For this term as well as for the terms telolecithal and centrolecithal I am indebted 

 to Mr Lankester. 



