BLASTOSPHERE AND MORULA. 65 



A. COMPLETE DIVISION Holoblastic Segmentation. 



I. Eggs with little and diffuse yolk material divide completely into 



approximately equal cells, 



[or, Ova which are alecithal (i.e., without yolk) undergo approxi- 

 mately equal holoblastic segmentation]. 



This is illustrated in most Sponges, most Ccelentera, some 

 "worms," most Echinoderms, some Molluscs, all Tuni- 

 cates, Amphioxus. and most Mammals. 



II. Eggs with a little yolk material accumulated towards one pole, 



divide completely, but into unequal cells, 



[or, Ova without very abundant deutoplasm, but with what they 

 have lying towards one pole (telolecithal), undergo unequal 

 holoblastic segmentation]. 



This is illustrated in some Sponges, some Ccelentera (e.g., 

 Ctenophora), some " worms," many Molluscs, the 

 lamprey, Ganoid Fishes, Ceratodus, 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 Elasmobranch and 

 Teleostean fishes, all Reptiles and Birds, and also in the 

 Monotremes or lowest Mammals. 



IV. Eggs with a considerable quantity of yolk, accumulated in a 



central core, and surrounded by the formative protoplasm, 

 divide partially, and superficially or peripherally, 

 [or, Ova which are centrolecithal undergo meroblastic and super- 

 ficial segmentation]. 



This is illustrated by almost all Arthropods, and by them 

 alone. 



Summarising the above, we have : 



C I T^V.,,,-,1 



A. Complete Division. jr* 



B. Partial Division. 



/ III. Discoidal. 

 \ IV. Peripheral. 



Blastosphere and Morula. The result of the 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 redivide, they often leave a large 

 5 



