CELL FORMATION BY UNION. 



45 



instead of being independent, as in the previous case. At 

 the time of union the filaments approach one another and lie 

 nearly parallel ; protuberances grow out from the contiguous 

 cells (Fig. 36, a, V) ; their extremities meet, and the walls are 

 absorbed, making a channel of communication from cell to 

 cell (Fig. 3G). Through this channel the protoplasm from 

 one of the cells passes into the cav- 

 ity of the other ; the two masses 

 unite and form a round or ovoid 

 cell, which soon secretes a wall of 

 cellulose (Fig. 37, A, b, and B, c). 



The particular kind of union in which 

 the two cells are of equal or nearly 

 equal size, and illustrated above by Cos- 

 marium and Spirogyra, lias received the 

 name of Conjugation. It is character- 

 istic of one fjroup of the Thallophytes, 

 viz., the Zygosporece. 



59. In Vaucheria, a fresh-wa- 

 ter Thallophyte, we have an ex- 

 ample of the union of cells of very 

 different sizes. The larger cells 

 (called oospJieres) are in lateral 

 protuberances of the large single 

 cell which composes the whole 

 plant (Fig. 38, A, and B, og}. The 

 protoplasm in these is of a spheri- 

 cal form, and is much denser than 

 in the main cell, from which .it is 

 separated in each case by a trans- 

 verse wall (shown in F). The 

 smaller cells (the sperm atozoids) 



-, j v ,-1 , i 11 ances from the contiguous rells 



are produced by the internal cell- approaching each other, x sso. 

 division of the protoplasm of simi- A ' 

 lar protuberances (the aiitheridia, A, and B, a). They are 

 very small as compared with the oospheres, and are naked 

 masses of protoplasm provided with two cilia, by means of 

 which they are locomotive (D). Upon escaping into the 

 water by the bursting of the old wall, they swim about, and 



Fig. 36. Two filaments of Spire- 

 ffyra longata about to conjugate ; 

 at a and b are seen the protuber- 



