BLASTOSPHERE AND MORULA. 65 



A. CoiipLETF, Division— Holol)lastic Segmentalion. 



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 jMolluscs, all Tuni- 

 cates, Ainpliioxiis. and most Alammals. 



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 Cielentera (c.i;.. 

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

 lamprey, CJanoid Fishes, Cerato.itis, Amphibians. 



B. Partial Division— Meroblastic .Segmentation. 



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



protoplasm lies as a small tlisc at one pole, divide jiartially, 

 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 

 Monotreraes 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 :- 

 A. Complete Di\'ision. 



I. Equal. 

 II. Unequal. 



B. Partial Division. [ I"' l^i=^™i''=^l-, 



( I v. Peripheral. 



Blastosphere and A'lorula. — 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 



