,\U.)DH-ICJA'110JN OF 0\'UM 



547 



the germinal vesicle. An investing cuticular membrane may 

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

 ah'eady 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 tnicropyle 

 (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 3 



ic. 137. — 0\'um of a Sea-urchin {Toao- 

 fineustes Ih'idnsX showing the radially- 

 ^triatetl cell-wall (vitelline membrane), 

 the protoplasm containing yolk granules 

 (vitellus), -the large nucleus (germinal 

 vesicle) with its network of chromatin, 

 antl a large nucleolus (germinal spot). 

 (From Balfour, after Hertwig.) 



