52 



BRITISH CHAROPHYTA. 



ovoidal form. The lateral walls of the spiral cells 

 subsequently break down, leaving the outer walls to 

 form a single integument marked by spiral furrows 

 along the lines of union. 



Incased and protected by the enveloping spiral 

 cells the oosphere consists of a nucleated mass of 

 protoplasm enclosed by a thin and extremely delicate 



FIG. 22. Development of oogonium of Nitella flexilis (after Sachs). 

 a, oosphere ; b, node-cell ; c, enveloping cells ; d, upper, and d 1 , 

 lower cells of coronula ; e, turning cell. i. Early stages of the 

 oogoiiium showing oosphere with turning cells at its base, and 

 enveloping cells with upper part cut off by double septum forming 

 two-staged coronula. ii. Further stage of development, iii. The 

 enveloping cells assuming well-defined spiral growth, and turning 

 cells divided by two septa. 



membrane. Large numbers of starch-granules and 

 oil-drops are developed in the protoplasm. 



Turning cells. 



At the base of the oogonia of the NiteUse 



are found three small cells called by 

 Braun "turning cells' (Wendungszellen). The forma- 

 tion of the first of these cells occurs at an early stage 

 of the oosphere's development and originates at its 



