76 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS 



develop {merogony), but the half containing only the ovum 

 nucleus degenerates. 



It should be noted that the chromosomes of the spermatozoon do not 

 fuse with the chrom.osomes of the ovum when fertiUsation occurs. 

 They are associated together and divide together in all the cell- 

 • divisions, whether of body-making or of the germ-cell lineage. In 

 some of the divisions of the germ-cell lineage there seems to be an 

 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 ex- 

 hibited 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 con- 

 trast 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, however, 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 



(i) Eggs with little and diffuse yolk material di\ide 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 (Fig. 39 

 (i)), some " Worms," most Echinoderms, some Molluscs, 

 all Timicates, Amphioxiis, 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 Coelentera {e.g. 

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

 rey. Ganoid Fishes, Dipnoi, Amphibians (Fig. 39 (2)). 



