SEGMENTATION. 65 



interesting interchange or "crossing over" of pieces of the members of 

 a pair of chromosomes. A special chromosome of the germ-cells seems 

 often to have to do with sex, whether as determiner of maleness or 

 femaleness, or as an index of these two physiological conditions. 



Segmentation. The different modes of division exhibited 

 by fertilised egg-cells depend in great measure on the 

 quantity and disposition of the passive and nutritive yolk 

 material, which is often called deutoplasm, in contrast to 

 the active and formative protoplasm. The pole of the ovum 

 at which the formative protoplasm lies, and at which the 

 spermatozoon enters, is often called the animal pole ; the 

 other, towards which the heavier yolk tends to sink, is called 

 the vegetative pole. In the floating ova of some fish, how- 

 ever, the yolk is uppermost, and the embryonic area 

 lowest. 



In contrasting the chief modes of segmentation, it 

 should be recognised that they are all connected by 

 gradations. 



A. COMPLETE DIVISION Holoblastic Segmentation 



1 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 Gcelentera (Figs. 

 31 (i) and 32), 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, Dipnoi, Amphibians (Fig. 31 (2)). 



B. PARTIAL DIVISION. Meroblastic Segmentation 



(3) Eggs with a large quantity of yoke 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. 31 (3) ), 



and also in the Monotremes or lowest Mammals. 



