90 CENTROLECITHAL SEGMENTATION. 



accepted,) that the part of the ovum of meroblastic ova which 

 does not segment is of the same nature as that which does segment, 

 and differs only in being relatively deficient in active protoplasm. 



Tlie following forms have meroblastic ova of the first type : the Cephalo- 

 poda, Pyrosoma, Elasmobranchii, Teleostei, Reptilia, Aves, Ornithodel- 

 phia (?) The second type of meroblastic segmentation occurs in many 

 Crustacea, (parasitic Copepoda, Isopoda Mysis, etc.). It is also stated to be 

 found in Scorpio. 



The ova of the majority of groups in the animal kingdom segment 

 according to one of the types which have just been described. These 

 types are not sharply separated, but form an unbroken series, 

 commencing with the ovum which segments uniformly, and ending 

 with the meroblastic ovum. 



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 concen- 

 trated at one pole of the ovum. When only a moderate quantity 

 of food-yolk is present the pole at which it is concentrated 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 seg- 

 mentation nearly parallel to the telolecithal group. This group takes 

 its start from the alecithal ovum as do the telolecithal 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 ceutrolecithal. It is especially characteristic of the Arthro- 

 poda, if not entirely confined 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 Paige mon (Bobretzky) and Peneus (Haeckel). A type 



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

 to Mr Lanke-ster. 



