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ESSENTIALS OF BIOLOGY 



as spherical and we orientate it as we do a terrestrial globe. 

 Actually the first division-plane of the segmenting ovum is usually- 

 determined by the place at which the sperm-head enters it in the 

 act of fertilization. We shall say that this plane is " meridional," 

 and that the second plane is also meridional and perpendicular 

 to the first one. The third plane is again perpendicular to the 

 former two planes, or is equatorial. Diagrammatically these 

 division-planes may be represented as follows : 



1 



^X/x^stode?^^ 



Fig. 26. — Segmentation of the Ovum. 



I, The undivided ovum; 2, 2-blastomere stage, there is one meridional division-plane; 

 3, the 4-blastomere stage, there are two meridional division-planes at right angles to each 

 other ; 4, the 8-blastomere stage, there are two meridional division-planes (as in 3) and one 

 equatorial division-plane at right angles to the two former ones ; 5, 8-blastomere stage in a 

 yolky egg ; 6, the blastoderm in a large-yolked egg. 



Close approximations to the above mode of segmentation occur 

 very often and in widely different groups of animals and we regard 

 it as the typical mode. (But the process may differ greatly from 

 the scheme.) Usually the blastomeres are of nearly equal size 

 up to this 8- cell stage, but after it there are, as a rule, two or 

 more " tiers " of blastomeres — the " lower " and larger ones, 

 and the '' upper " or smaller ones. This is always so when the 

 ovum contains much food-yolk (No. 5, Fig. 26). 



In the extreme case, that of the large-yolked eggs of Teleostean 



