It is convenient to distinguish the ova which segment uniformly by some term; and I should propose for this the term alecithal[56], 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 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 segmentation 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 centrolecithal. It is especially characteristic of the Arthropoda, 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 Palæmon (Bobretzky) and Penæus (Hæckel). 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 till rather late in the segmentation. More irregular examples of unequal segmentation are also afforded by other Crustaceans, e.g. various members of the genus Chondracanthus (Beneden and Bessels) and by Myriapods. In all these cases segmentation ends in the formation of a layer of cells enclosing a central mass of food-yolk.
Fig. 48. Segmentation of a Crustacean ovum (Penæus).
(After Hæckel.)
The sections illustrate the type of segmentation in which the yolk is aggregated at the centre of the ovum.