436 



HISTOLOGY 



FIG. 392. Youngest stage of the four pollen cells of 

 Magnolia, which are all inclosed in the single cell- 

 wall made by the pollen mother cell. X 1800. 



original cell-wall which still incloses the four, they lie massed in the sac 

 to await its ripening and rupture. The drying up of the scant remains 



of the former nurse cells sets 

 them entirely free from any 

 connection with the pollen 

 sac. 



The spermatogenesis of the 

 skate, Raja ocellata, affords 

 a very splendid object for 

 demonstrating most of the 

 details of sperm formation 

 during which reduction takes 

 place in the male gametes of 

 animals. It illustrates at the 

 same time several interesting 

 histological methods of ar- 

 rangement. The testis is a 

 solid mass of tissue devel- 

 oped from the genital ridge, 

 which is practically the same as in Acanthias (see Fig. 380). The 

 large primitive reproductive cells of this ridge have multiplied and are 

 situated, in the adult Raja, at a number of positions near the surface of 

 the testis. They remain in these positions during the life of the animal 

 and are constantly dividing so that they form small separate groups of 

 germ cells which we shall call the germinal centers. In this center are 

 a certain number of connective-tissue cells as well as the reproductive 

 cells. 



Each of these primitive reproductive cells is surrounded by several 

 of the connective-tissue cells (Fig. 393). 

 On the boundaries of the germinal center 

 the reproductive cells may be seen divid- 

 ing inside their enlarging connective-tis- 

 sue cell coverings as in Figure 394, where 

 each of the two groups of cells shown 

 came from a single reproductive cell such 

 as is shown in Figures 380 and 393. Thus 

 each reproductive cell comes to form a 

 spermatic lobule, which grows in size and 

 matures its contents as it is pushed away 

 from the germinal center by the forma- 

 tion of new lobules from this center. This 

 whole mass of many lobules is surrounded by a capsule of connective 

 tissue, forming one of the lobes of the testis. At the periphery of 



FIG. 393. Two primitive repro- 

 ductive cells (pre-spermatogonia) 

 in their resting stage in the adult 

 testis of Scyllium. Each is sur- 

 rounded by a few connective-tissue 

 cells. (After MOORE.) 



