THE SEGMENTATION OF THE OVUM. 



91 



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 segmen- 

 tation 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. 



The peculiarity of the centrolecithal ova with regular or unequal 

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

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



FIG. 48. SEGMENTATION OF A CRUSTACEAN OVUM (PENEUS). (After Haeckel.) 



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

 the centre of the ovum. 



yk central yolk mass. 



1 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. 



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. Thus in 

 Eupagurus prideauxii 1 (fig. 49), and probably in the majority 

 of Decapods, the egg is divided successively into two, four and eight 

 distinct segments, and it is riot till after the fourth phase of the 



1 Mayer, Jenaiache Zcitschrift, Vol. xi. 



