68 STRUCTURE OF THE EGG 



An investing membrane may or may not be present. In 

 other words the egg is a cell : it is convenient, for reasons 

 which will appear immediately, to speak of it as the ovum 

 or egg-cell. 



The young or immature ova of all animals present this 

 structure, but in many cases certain modifications are under- 

 gone before the egg is mature, i.e., capable of development 

 into a new individual. For instance, the protoplasm may 

 throw out pseudopods, the egg becoming amoeboid (see 

 Fig. 53) ; or the surface of the protoplasm may secrete a thick 

 cell-wall (see Fig. 61). The most extraordinary modification 



FIG. 12. A, ovum of an animal (Carmarina hastata, one of the 

 jelly fishes), showing protoplasm (gd), nucleus (gv], and nucleolus (gm). 



B, ovum of a plant (Gymnadenia conopsea, one of the orchids), 

 showing protoplasm (plsni), nucleus (mi), and nucleolus (mi 1 ). 



(A, from Balfour after Haeckel : B, after Marshall Ward. ) 



takes place in some vertebrata such as birds. In a hen's 

 egg, for instance, the yolk-spherules 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, 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, however large and 



