SEGMENTATION. 59 



This is illustrated in some Sponges, some Coelen- 

 terates {e.g.., Ctenophora), some " worms," 

 many Molluscs, in the lamprey, in Ganoid 

 Fishes, in Amphibians. 



B. Partial Division — Meroblastic Segmentation. 



III. Eggs with a large quantity of yolk, on which the 

 formative protoplasm lies as a small disc at one pole, 

 divide partially, and in discoidal fashion, 



[or, Ova which are telolecithal, and have a large 

 quantity of deutoplasm, undergo meroblastic and 

 discoidal segmentation]. 



This is illustrated in all Cuttle-fishes, all Elasmo- 

 branch and Teleostean fishes, all Reptiles and 

 Birds, and also in the Monotremes or lowest 

 Mammals. 



IV. Eggs with a considerable quantity of yolk, accumu- 

 lated in a central core, and surrounded by the 

 formative protoplasm, divide partially and superficially 

 or peripherally, 



[or. Ova which are centrolecithal undergo meroblastic 

 and superficial segmentation]. 



This is illustrated by almost all Arthropods, and 

 by them alone. 



Summarising the above, we have : — 



A. Complete Division. | j}; g^^^^^^^j 



B. Partial Division. < 



III. Discoidal. 



IV. Superficial. 



Blastosphere and Morula. — The result of division is usually 

 a ball of cells. But when the yolk is very abundant (III.) 

 a disc of cells — a discoidal blastoderm— is formed at one pole 

 of the mass of nutritive material which it gradually surrounds. 



As the cells divide and re-divide, they often leave a spacious 

 central cavity, and a hollow ball of cells— a blastosphere or 

 blastula — results . 



