262 GEO. W. TANNREUTHER. 



the hydra, are about uniform in size, shape and appearance. The 

 formation of the ovary first becomes recognizable by a rapid 

 growth of the interstitial cells in some special region and not by 

 their more rapid division. After these cells have increased several 

 times in volume, they become differentiated into two distinct 

 regions, namely, a more central region, which gives rise to the 

 ovum or ova, and a peripheral region which may be considered 

 the temporary ovary, whose cells cease to enlarge and later con- 

 tribute directly to the formation of the yolk (PI. VIII., Fig. 2). 

 The ectodermal cells, with their scanty supply of cytoplasm and 

 nuclei that stain very faintly, are pushed more to the exterior by 

 the enlarged interstitial cells. They often remain connected with 

 the mesoglea by fibrous strands. 



The cells of the central region vary in number and their con- 

 tents contribute directly to the formation of the ovum or ova 

 (Fig. 2, a, b, c). Not merely one but all of these cells continue 

 to enlarge. This increase in size affects the nuclei as well as 

 the cell bodies. The cell walls break down and the cytoplasm 

 which now becomes a common multinucleate mass without any 

 definite outline comes to lie between the enlarged cells of the 

 peripheral region of the ovary and the mesoglea (Fig. 3). The 

 egg at this stage of development as stated above is multinucleate. 

 All of the nuclei enlarge somewhat, the chromatin assumes the 

 spireme condition and the nucleoli are very prominent. One of 

 these nuclei, seldom more than one (Fig. 3, a), continues to en- 

 large and becomes the egg nucleus. The remaining nuclei 

 gradually break down and disappear within the cytoplasm. When 

 two nuclei persist, the cytoplasm becomes separated into two 

 distinct parts. Each part with its contained nucleus becomes a 

 separate egg, which develops independently of the other. The 

 egg does not begin as a single cell, but as a multinucleate mass 

 which results from the fusion of several cells after the breaking 

 down of their walls. Thus, the cytoplasm which originates from 

 several cells becomes the cytoplasm of the egg. Up to this stage 

 of development it is very difficult to determine the exact origin 

 of the sexual organs except from a study of sections. The 

 ovaries and spermaries both begin by a rapid growth of the in- 

 terstitial cells, but in the formation of the spermary, when the 



