64 REPRODUCTION AND LIFE HISTORY. 



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 Coelentera (Figs. 

 29 (I ) and 30), some " worms," most Echinoderms, some 

 Molluscs, all Tunicates, Amphioxus, and most Mammals. 



(2) Eggs with considerable yolk material accumulated towards one 



pole, divide completely, but into unequal cells, 

 [or, Ova with a considerable amount of deutoplasm lying towards 

 one pole (telolecithal), undergo unequal holoblastic segmenta- 

 tion]. 



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

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

 rey, Ganoid Fishes, Ceratodtis, Amphibians (Fig. 29 (2)). 



B. Partial Division — Meroblastic Segmentation. 



(3) 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 (Fig. 29 (3)), 

 and also in the Monotremes or lowest Mammals. 



(4) 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 most Arthropods (Fig. 29 (4)), and 

 by them alone. 



Blastosphere and morula. — The result of the division is 

 usually a ball of cells. But when the yolk is very abundant 

 (3), 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 

 central cavity — the segmentation cavity — and a hollow ball 

 of cells — a blastosphere or blastula — results. 



But if the so-called " segmentation cavity " be very small 

 or absent, a solid ball of cells or morula, like the fruit of 

 bramble or mulberry, results. 



Gastrula. — The next great step in development is the 



