THE SEGMENTATION OF THE OVUM. 



Ill 



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 Chondr acanthus (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. 



The peculiarity of the centrolecithal ova with regular or un- 

 equal segmentation is that (owing to the presence of the yolk in 

 the interior) the furrows which appear on the surface are not 



FIG. 48. SEGMENTATION OF A CRUSTACEAN OVUM (PEN^EUS). (After Hseckel.) 



The sections illustrate the type of segmentation in which the yolk is aggregated at 

 the centre of the ovum. 



yk. central yolk mass. 



i and 2. Surface view and section of the stage with four segments. . In 2 it 

 is seen that the furrows visible on the surface do not penetrate to the centre of the 

 ovum. 



3 and 4. Surface view and section of ovum near the end of segmentation. The 

 central yolk mass is very clearly seen in 4. 



continued to the centre of the egg. The spheres which are thus 

 distinct on the surface are really united internally. Fig. 48, 

 copied from Haeckel, shews this in a diagrammatic way. 



Many ova, which in the later stages of segmentation exhibit 

 the characteristics of true centrolecithal ova, in the early stages 

 actually pass through nearly the same phases as holoblastic ova. 



