52 



BRITISH CHAEOPHYTA. 



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 



Pig. 22. — Development of oogonium of Nitella flexilis (after Sachs). 

 o, oosphere ; h, node-cell ; c, enveloping cells ; d, upper, and. d', 

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

 oogonium showing oosphere with turning cells at its base, and 

 enveloping cells with upper part cut off by double septum forming 

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

 enveloping cells assuming well-defined spiral growth, and turning 

 cells divided by two septa. 



starch-granules 



and 



membrane. Large numbers of 

 oil-drops are developed in the protoplasm. 



. n ^*' ^^^^ ^^^® *^^ ^^^ oogonia of the Nitellse 

 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 



