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TEXT-BOOK OF EMBRYOLOGY. 



later utilized in the growth and increase of the embryonic cells. The food 

 granules— deutoplasm— are suspended in the cytoplasm. The distribution, 

 however, of these granules in the human ovum is not uniform; a mass of them 

 being found in the center of the cell surrounding the nucleus, while an almost 

 clear zone of cytoplasm forms the periphery of the cell. 



The nucleus of the ovum occupies a position near the center within the 

 deutoplasm mass, though in the ovum of a mature Graafian follicle it is almost 

 invariably slightly eccentric. It is large proportionately as the ovum is large. 

 Its structure does not differ essentially from that of any other nucleus. There 

 is a distinct nuclear membrane enclosing the usual nuclear structures — the 

 nuclear liquid, the network of chromatin, the achromatic network and a single 



nucleolus or germinal spot (p. 2, Fig. 1). In 

 a fresh human ovum amoeboid movements 

 have been observed in the nucleolus. The 

 significance of the nucleolus is as little known 

 as in any other cell. 



A centrosome, though it may be present, 

 has not been observed in the human ovum. 



The amount and distribution of the yolk 

 granules have an important bearing upon the 

 changes which take place in the egg subse- 

 quent to fertilization and have led to the 

 classification of eggs as alecithal, telolecithal 

 and centrolecithal. Alecithal eggs (Pig. 5) are 

 those in which the yolk granules are fairly 

 evenly distributed throughout the cytoplasm 

 (Amphioxus, most Mammals, includingman). 

 Telolecithal eggs (Figs. 6 and 7) are those in 

 which the yolk is'in excess at one pole, the 

 cytoplasm at the opposite pole (Amphibians, Reptiles, Birds). Centrolecithal 

 eggs are those in which a central yolk mass is surrounded by a compara- 

 tively thin layer of cytoplasm (Arthropods). (For further description see 

 Cleavage, page 42.) 



Telolecithal ova show a condition known as polar differentiation. By 

 polar differentiation is meant the more or less complete separation of cytoplasm 

 and deutoplasm, so that the cytoplasm is present in excess at one pole of the egg 

 and the deutoplasm in excess at the opposite pole. The frog's egg is a familiar 

 example of this differentiation, the dark side of the egg indicating an excess of 

 cytoplasm. Inasmuch as deutoplasm is generally heavier than cytoplasm, an 

 egg with polar differentiation, if left free to revolve, as in water, will assume a 

 definite position with the protoplasmic or animal pole above and the deuto- 



Fig. 6.— Semidiagrammatic representa- 

 tion of ovum of frog (Rana sylvatica). 

 The dark shading represents the cyto- 

 plasmic pole, the light shading immedi- 

 ately below represents the deutoplasmic 

 pole. The light shading around the 

 ovum represents the gelatinous sub- 

 stance (secondary egg membrane). 



