5 6 4 



STRUCTURE OF OVUM 



In other words the egg, as we have already seen,'is*a cell. 

 An investing cuticular membrane is usually present. 



The young or immature ova of all animals present this 

 structure, but in many cases certain modifications are 

 undergone before the egg is fully formed. For instance, the 

 protoplasm may throw out pseudopods, the egg becoming 

 amoeboid (p. 307) ; or, as 

 mentioned above, the sur- 

 face of the protoplasm 

 may secrete a membrane 

 or cell- wall often of con- 

 siderable thickness, and 

 known as the vitelline 

 membrane (p. 196 and Fig. 

 147), which may be per- 

 forated at one pole by an 

 aperture, the micropyle (p. 

 415 and Fig. 150, B, c). 

 The most extraordinary 

 modification takes place 

 in some Vertebrata, such 

 as dogfishes (p. 470) and 

 birds. In a hen's egg, for instance (Fig. 148), the yolk- 

 granules increase immensely, swelling out the microscopic 

 ovum until it becomes what we know as the " yolk " of the j 

 egg : around this layers of albumen or " white " are de- 

 posited by the glands of the oviduct, and finally the shell- 

 membrane and the shell. Hence we have to distinguish 

 carefully in eggs of this character between the entire " egg " 

 in the ordinary acceptation of the term, and the ovum or 

 egg-cell. But complexities of this sort do not alter the 

 fundamental fact that all the higher animals begin life as 

 a single cell ; or in other words, that multicellular animals, 



FIG. 147. Ovum of a Sea-urchin (Toxo- 

 pneustes lividus\ showing the radially- 

 striated cell-wall (vitelline membrane), 

 the protoplasm containing yolk-granules 

 (vitellus), the large nucleus (germinal 

 vesicle) with its network of chromatin, 

 and a large nucleolus (germinal spot). 

 (From Balfour's Embryology ; after 

 Hertwig.) 



