12 



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 



nudeolus or germinal spot (p. 2, Fig. i). In 

 /-^ a fresh human ovum amoeboid movements 



have been observed in the nucleolus. The 



\ 



\ significance of the nucleolus is as little known 

 ^jjSiJfc^ 5| as m anv other cell. 



K A centrosome, though it may be present, 



has not been observed in the human ovum. 



A classification of ova has been made 

 on the basis of the amount and distribution 

 of the yolk; conditions which strongly affect 

 the subsequent processes of development. 

 The term meiolecithal is used to designate 

 ova in which the yolk granules are relatively 

 few (ova of Amphioxus, most Mammals in- 

 cluding man). Mesolecithal ova are those 

 which contain a moderate amount of yolk 

 (Amphibia.) Ova which contain a relatively 

 large amount of yolk are classed as foly- 



lecithal (Reptiles and Birds). In meiolecithal eggs the yolk granules are as a 

 rule evenly distributed through the cytoplasm. In mesolecithal and polyleci- 

 thal eggs, on the other hand, the yolk is unevenly distributed, giving rise to a 

 condition known as polar differentiation ; the protoplasm is in excess at one pole 

 of the egg and the deutoplasm in excess at the opposite pole. Such ova are 

 spoken of as telolecithal. The frog's egg is a familiar example of this differ- 

 entiation, the dark side of the egg indicating an excess of cytoplasm. Inasmuch 

 as deutoplasm is generally heavier than cytoplasm, an egg with polar differ- 

 entiation, if left free to revolve, as in water, will assume a definite position 

 with the protoplasmic or animal pole above and the deutoplasmic or vegeta- 

 tive pole below. An exception to this is found, however, in the pelagic teleost 

 eggs, which float with the deutoplasmic pole upward. 



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). 



