THE GAMETES IN PLANTS AND ANIMALS 



341 



The interesting o5genesis of Sternaspis scutata, a peculiar marine 

 annelid, is shown in Figs. 184 and 185. The eggs arise from cells 

 on the walls of certain blood vessels and as they grow develop a 

 stalk containing a loop of the blood vessel, so that blood flows 

 directly through the 

 basal end of the egg. 

 Fig. 184 shows the egg 

 at the beginning of yolk 

 formation: the cyto- 

 plasm contains a few 

 yolk granules and shows 

 a strongly radiate 

 structure centering 

 about the vascular loop. 

 In the full-grown egg 

 the cytoplasm is loaded 

 with numerous large 

 yolk spheres (Fig. 185) 

 except at the basal end, 

 where there is an area 

 of granular cytoplasm. 

 At this stage the egg 

 becomes free from the 

 stalk, which undergoes 

 atrophy and resorption. 



A different type of 

 oogenesis is shown in 

 Fig. 186, an ovarian 

 tubule from the water 

 beetle Dytiscus margi- 

 nalis. Here growing 

 eggs alternate with groups of so-called nurse cells, which serve as a 

 food supply and are used up during the growth of the egg. 



Three stages of ascidian oogenesis are shown in Figs. 187-89. 

 The first, the young ovotestis, the animals being hermaphroditic, 

 with a young egg cell at the left, the second, the growing egg sur- 

 rounded by its follicle from which the so-called test cells — -cells 



Fig. 175. — Female gametophyte of Torreya, a 

 conifer, showing the egg, o, and above it the pollen 

 tube with the two male nuclei, sp. From Coulter 

 and Land, '05. 



