COELENTEEATA 37 



constricting. The budded hydra floats away from the parent, 

 and new individuals are thus produced and liberated in great 

 numbers. 



Sex cells are formed usually at the approach of winter. 

 Some individuals are, or appear to be, practically males, but 

 the usual condition is that sperm cells and ova are formed in 

 succession in the ectoderm of the same individual. The 

 interstitial cells are the seat of origin in each case. 



Near the tentacular region of the column one or several 

 groups of interstitial cells begin to divide, forming in this 

 way a large number of cells distending the epithelium cells 

 about them into a cone-like hump, called the testis. At the 

 end of the period of cell division the cells become converted 

 into flagellate spermatozoa, which are liberated through an 

 opening at the summit of the cone-shaped swelling. 



Lower down the column, and usually, as has been said, at 

 a later period, a group of interstitial cells yields an ovum 

 which grows by developing pseudopodia and feeding upon the 

 other interstitial cells. A swelling styled the ovary is thus 

 produced, which projects out as a tumour on the side of the 

 column. 



When fully grown the ovum becomes rounded in the cavity 

 containing it, and two polar bodies are budded off from its 

 outer surface ; thereafter the egg pushes its way through 

 the ectodermal cells of the ovary and, anchored by these cells, 

 projects into the water. Spermatozoa are attracted by the 

 ovum and, one of them gaining admission, fertilisation takes 

 place. The ovum then divides repeatedly and a sphere of a 

 single layer of cells is produced. This stage of development is 

 called the bias tula, and the cavity is called the segmentation 

 cavity. The inner face of the single layer of cells buds off 

 cells which are to form the endoderm. These do not arise 

 from a definite region of the outer layer, but generally ; they 

 are said therefore to have a multipolar origin. At the same 

 time the ectoderm secretes a coat of mucus on its outer face, 

 and this is the prelude to the formation of two protective 

 coats, an outer horny shell and a more delicate membrane 

 internal to it. The process also involves the separation of the 

 embryo from the parent column and it falls to the bottom. 



