THE SEGMENTATION OF THE OVUM. 93 



yet, till they have been demonstrated, some doubt must remain 

 on the nature of these yolk spheres. It is probable that not 

 all the nuclei which result from the division of the first segmen- 

 tation nucleus become concerned in the formation of the super- 

 ficial blastoderm, but that some remain in the interior of the 

 ovum to become the nuclei of the yolk spheres. 



In Myriapods (Gldlognatha) a peculiar form of segmentation has been 

 observed by Metschnikoff 1 . The ovum commences by undergoing a perfectly 

 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 character- 

 izes 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 

 Chelifer segmentation takes place in nearly the same manner as in 

 Myriapods (fig. 50). 



FIG. 50. SEGMENTATION AND FORMATION OF THE BLASTODERM IN CHEUFER. 



(After Metschnikoff.) 



In A the ovuin is divided into a number of separate segments. In B a number of 

 small cells have appeared (bl) which form a blastoderm enveloping the large yolk 

 spheres. In C the blastoderm has become divided into two layers. 



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 



Zeitgchrift fiir 



Vol. xxiv. 1874. 



