340 



SENESCENCE AND REJUVENESCENCE 



the animal egg as a single cell involves so much more extensive 

 and conspicuous change than in the plant, it has attracted much 

 attention and the course of oogenesis has been described for many 

 animal species. The following figures include characteristic stages 

 in the differentiation of a few animal eggs. Figs. 178-80 show 

 the egg of the fresh-water hydra, first at the beginning of its 

 growth as a small cell lying between the cells of the ectoderm 

 (Fig. 178); secondly, as a large amoeboid cell in the ovary 



(Fig. 179); and, thirdly, 

 as a full-grown egg, still 

 in the ovary, with large 

 yolk spheres in the 

 cytoplasm. Figs. 181 

 and 182 show the primi- 

 tive germ cells and the 

 final stage of oogenesis 

 in the liver fluke Fasciola 

 hepatica, a parasitic flat- 

 worm. In most of the 

 flatworms the egg 

 accumulates little or no 

 yolk within its own 

 cytoplasm, but other 

 nutritive cells contain- 

 ing yolk are inclosed in 

 the capsule with it be- 

 fore it is extruded. In 

 these forms the egg cell 

 itself remains of small 

 size and its growth history is relatively simple. In this and in 

 various other animals the egg as it grows develops a stalk (Fig. 182) 

 by which it is connected with the ovarian wall and through which 

 it probably receives most or all of its nutrition. Fig. 183 shows 

 an ovary of the bryozoan Plumatellafungosa, with eggs in various 

 stages of growth and differentiation. These eggs develop succes- 

 sively from the primitive cells, and each egg in turn is displaced 

 by the growth of another behind it. 



174 



FIGS. 173-174. Fig. 173, archegonium of Nephro- 

 dium, a fern, containing the egg, o (from Yama- 

 nouchi, '08); Fig. 174, fertilized egg of Zamia, 

 a cycad (from Webber, '01). 



