4 THE BIOLOGY OF HYDRA : 1961 



Thus, the constituent cells of Hydra can be considered as embedded 

 in mesoglea and the mesoglea forms a supporting substance for 

 the cells. 



THE ENDODERM 



Gland cells (Figs. 11, 12). The gland cell pours its secretion 

 into the lumen to break down the food and make its products avail- 

 able for digestion. Essentially only one kind of gland cell has been 

 found. This cell contains a series of large interlacing vacuoles, 

 which most frequently appear light, but sometimes dark. Toward 

 the base of the cell, the vacuoles frequently are smaller than in 

 the portion of cell near the lumen. The cell appears to be under- 

 going a process of manufacture of the vacuoles starting toward the 

 base. Thus, various vacuolar arrangements can be seen, but they 

 are believed to be stages in the appearance of a single kind of gland 

 cell. Between the vacuoles, some mitochondria and double mem- 

 branes appear. Toward the base of the cell, the vacuoles are not 

 present and the cytoplasm is filled with mitochondria and double 

 membranes. It is probably here where the manufacture of the 

 vacuolar contents, which will be secreted into the lumen, begins. 

 The nucleus of the cell is toward its base. This cell apparently does 

 not rest upon the mesoglea. 



The digestive cell (Figs. 13-16). The digestive cell absorbs 

 the food products after action of the gland cell. The digestive cell 

 also undergoes cyclical changes according to the feeding activities 

 of Hydra and also contains various inclusions depending on the kind 

 of food and time of feeding. The cell can appear columnar and 

 rather well organized or can contain huge vacuoles. It has a light 

 cytoplasm with mitochondria and a Golgi apparatus. The surface 

 of the digestive cell usually has a series of small cytoplasmic projec- 

 tions or villi extending into the lumen (Fig. 14). The digestiv^e 

 cells contain the endodermal muscle filaments at their base and 

 rest upon the mesoglea (Figs. 15, 16). 



Flagella (Fig. 18). Apparently both gland cell and digestive 

 cell have flagella. It is difficult to determine exactly how many 

 project from each cell. Two to four flagella are commonly seen. 

 The flagella present the nine peripheral and two central longitudinal 



