vi STRUCTURE OF THE EGG 69 



into a new individual. For instance, the protoplasm may 

 throw out pseudopods, the egg becoming amoeboid (see 

 Fig. 52) ; or the surface of the protoplasm may secrete a thick 

 cell-wall (see Fig. 61). 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 (Carwarina Iiastata, one of the 

 elly fishes), showing protoplasm (gif), nucleus (gv], and nucleolus ("<) 



B, ovum of a plant (Gytnnadei/ia conopsea, one of the orchids), showing 

 protoplasm (/>/sw), nucleus (////), and nucleolus (mi). 



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



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

 f in other words that multicellular animals, however large and 

 i complex they may be in their adult condition, originate as 

 \ unicellular bodies of microscopic size. 



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

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

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

 curately megasporangia see Lesson XXXIV., and Fig. 127), 

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



