114 CENTROLECITHAL SEGMENTATION. 



observed by Metschnikoff 1 . The ovum commences by undergoing a per- 

 fectly normal, though rather irregular total segmentation. But after the 

 process of division has reached a certain point, scattered masses of very 

 small cells make their appearance on the surface of the large spheres. These 

 small cells have probably arisen in a manner analogous to that which 

 characterizes the formation of the superficial cells of the blastoderm in the 

 types of centrolecithal ova already described. They rapidly increase in 

 number and eventually form a continuous blastoderm; while the original 

 large segments remain in the centre as the yolk mass. In the interesting 

 Arachnid Cfieltfer segmentation takes place in nearly the same manner as in 

 Myriapods (fig. 50). 



It is clear that it is not possible in centrolecithal ova to have 

 any type of segmentation exactly comparable with that of 

 meroblastic ova. There are however some types which fill the 

 place of the meroblastic ova in the present group, in as tmich as 

 tJiey are characterised by tfie 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 



FIG. 51. FOUR SUCCESSIVE STAGES IN THE SEGMENTATION OF THE EGG OF TETRA- 

 NYCHUS TELARIUS. (After Clapnrede.) 



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 



1 Zeitschrift fur whs. Zool., Vol. XXIV. 1874. 



