THE DEVELOPMENT OF HYDRA. 263 



interstitial cells have increased two or three times in volume, they 

 begin to divide mitotically and give rise to the spermatogonia. 

 In case of the ovary, on the other hand, there is no division of 

 the interstitial cells after they have begun to enlarge. 



As the egg nucleus enlarges, the cytoplasm sends out pseud- 

 opodial processes (Fig. 4), which form very rapidly and often 

 encircle the entire body of the hydra. 



According to Kleinenberg {H. viridis), in a zone which sur- 

 rounds half of the body of the hydra there appear between the 

 neuromuscle cells (ectoderm cells) small tongues of interstitial 

 cells, the nuclei of which are so closely pressed together, that it 

 is hard to distinguish between nucleus and cell body. He further- 

 more states that by a progressive multiplication of these cells the 

 neuromuscle cells are pushed aside and the tongues of interstitial 

 cells unite with each other, forming a single-layered oblong plate 

 of cells between the ectoderm and endoderm. When the ovary 

 has reached this stage of development, one of the cells, which is 

 situated near the middle of the oblong plate, grows much faster 

 than its neighbors and becomes the egg. The egg cell sends 

 out pseudopodia, which grow very rapidly between the cells of 

 ovary. After the pseudopodia have reached their maximum de- 

 velopment they are drawn in, and the egg is completely formed. 

 The cells surrounding the egg break down and act as food for it. 



The tongues of interstitial cells which Kleinenberg speaks of 

 are found not only in the region where the ovary begins, but in 

 other parts of the hydra as well. They are especially abundant 

 in hydra that are budding vigorously. The pseudopodia do not 

 grow out between the cells of the ovary, but rather between the 

 ovary as a whole and the mesoglea (Figs. 4 and 6, ps). 



During the growth of the egg the cytoplasm becomes vacuo- 

 lated. The nuclear membrane is very indistinct, but the nucleo- 

 plasm becomes very dense and granular (Fig. 4, a). Several of 

 the degenerating nuclei are still visible in the cytoplasm. In Fig. 

 5, a stage a little later, they have entirely disappeared. When 

 the egg has reached its growth, it is amoeboid in form with the 

 nucleus near the center. The egg at this stage of development 

 contains no yolk (Figs. 4 and 5), but when the pseudopodia are 

 completely formed, the nuclei of the interstitial cells forming the 



