CENTROLECITHA L SEGMENTATION. 



meroblastic ova in the present group, in as much as they ore 

 characterised by the presence of a large bulk of food-yolk which either 

 does not segment, or does not do so till a very late stage in the 

 development. The essential character of this type of segmentation 

 consists in the division of the germinal vesicle in the interior, or 

 at the surface of the ovum into two, four, etc. nuclei (fig. 51). These 

 nuclei are each of them surrounded by a specially concentrated layer 

 of protoplasm (fig. 51) which is continuous with a general protoplasmic 



Fig. 51. FOUR SUCCESSIVE STAGES IN THE SEGMENTATION OF THE EGG OF TETEANYCHITR 



. (After Claparede.) 



reticulum passing through the ovum [not shewn in fig. 51]. The yolk 

 is contained in the meshes of this reticulum in the manner already 

 described for other ova. 



The ovum, like that of Eupagurus before segmentation, is now a 

 syncytium. Eventually the nuclei, having increased by division and 

 become very numerous, travel, unless previously situated there, to the 

 surface of the ovum. They then either simultaneously or in succession 

 become, together with protoplasm around them, segmented off from 

 the yolk, and give rise to a peripheral blastoderm enclosing a central 

 yolk mass. In the latter however many of the nuclei usually 

 remain, and it also very often undergoes a secondary segmentation into 

 a number of yolk spheres. 



The eggs of Insects afford numerous examples of this mode of 

 segmentation, of which the egg of Porthesia 1 may be taken as 

 type. After impregnation it consists of a central mass of yolk which 

 passes without a sharp line of demarcation into a peripheral layer of 

 more transparent (protoplasmic) material. In the earliest stage 

 observed by Bobretzky there were two bodies in the interior of the egg, 



1 Bobretzky, 7. fit. /. //... Zonl, Bel. xxxi., 



