YI STRUCTURE OF THE EGG 69 



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

 cell-wall (see Fig. 6 1). The most extraordinary modification 

 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 



FIG. ii. A, ovum of an animal {Car marina hastata, one of the 

 jelly fishes), showing protoplasm (gd), nucleus (gv), andnucleolus (,^-w). 



B, ovum of a plant (Gymnadcnia conofsca, one of the orchids), 

 showing protoplasm (pis in), nucleus (nit), and nucleolus (mi). 



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



fact that all the higher animals begin life as a single cell, or 

 in other words that multicellular animals, however large and 

 complex they may be in their adult condition, originate as 

 unicellular bodies of microscopic size. 



The same is the case with rdl the higher plants. The 

 pistil or seed-vessel of an ordinary flower contains one or 

 more little ovoidal bodies, the so-called " ovules " (more 

 accurately megasporangia (see Lesson XXX., and Fig. 89), 

 which, when the flower withers, develop into the seeds. A 

 section of an ovule shows it to contain a large cavity, the 



