MODIFICATION OF OVUM 



547 



the germinal vesicle. An investing cuticular membrane may 

 or may not be present. In other words the egg, as we have 

 already seen, is a cell. 



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. I'or instance, the 

 protoplasm may throw out pseudopods, the egg becoming 

 amoeboid (p. 302) ; or, as 

 mentioned above and as is 

 usually the case, the sur- 

 face of the protoplasm 

 may secrete a cell-wall, 

 often of considerable thick- 

 ness, and known as the 

 vitelline membrane (p. 196 

 and Fig. 137), which may 

 be perforated at one pole by 

 an aperture, the micropyle 

 (p. 400). The most extra- 

 ordinary modification takes 

 place in some Vertebrata, 

 such as dogfishes (p. 454) 

 and birds. In a hen's egg, 

 for instance (Fig. 138), the yolk-granules increase immensely 

 swelling out the microscopic ovum until it becomes what 

 we know as the " yolk " of the egg : around this layers of 

 albumen or " white " are deposited 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 accep- 

 tation of the term, and the ovum or egg-cell. But com- 

 plexities of this sort do not alter the fundamental fact that 

 all the higher animals begin life as a single cell, or in other 



N N 2 



Fir,. 137.— Ovum of a Sea-urchin (T'o.ii;- 

 pneustes lividus\ showing the radially- 

 striated cell-wall (vitelline membrane), 

 the protoplaiim containing yolk granules 

 (vitellus), the large hucleus (germinal 

 vesicle) with its network of chromatin, 

 and a large nucleolus (germinal spot). 

 (From Balfour, after Hertwig.) 



