THE GAMETES IN PLANTS AND ANIMALS 



339 



egg of the cycad Zajnia; Fig. 175, the archegonium of a conifer, 

 Torreya taxifolia, containing the large egg: incidentally this figure 

 also shows the pollen tube 

 with the two small male 

 nuclei near the tip. The 

 development of the female 

 gamete in the angiospcrms 

 isoutlined in Figs. 141, A-E 

 (p. 321). Fig. 176 is the 

 embryo sac of the sun- 

 flower at the time of ferti- 

 lization, and Fig. 177, that 

 of the conefiower, another 

 composite, at the same 

 stage. The eggs in all 

 these plants are manifestly 

 highly specialized cells 

 which have undergone 

 great changes from the 

 embryonic condition. 



The animal egg usually 

 exhibits an even greater de- 

 gree of morphological 

 specialization than that of 

 the plant because it is 

 loaded to a greater or less 

 degree with granules or 

 masses of yolk substance 

 which becomes available 

 as a nutritive supply at 

 the beginning of embry- 

 onic development. The 

 accumulation of yolk is 

 often so great that the egg 

 cell attains an enormous 

 size, the bird's egg representing the extreme of devclopnienl in 

 this direction. Since the period of growth and differentiation of 



Figs. 168-172. — Peculiar forms of sjxjrma- 

 tozoa from the arthroi)ods: Figs. 16S, 169, 170, 

 Pinnotheres, Maja, and Miinidia, all Crustacea 

 (from Koltzofl, '06a); Figs. 171, 172, Acanio- 

 loplitis, A galena, both spiders (from Boscnbcrg, 

 '05). 



