28 CCELENTERATA. 



described, mainly in the fact that they have a proportionately 

 smaller amount of protoplasm round the germinal vesicle. 

 They grow to a considerable extent at the expense of germinal 

 cells which do not become converted into ova. 



The ova of many Ccelenterata undergo changes of a more 

 complicated kind before attaining their full development. Of 

 these ova that of Hydra may be taken as the type. The ovary 

 of Hydra (Kleinenberg, No. 9) is constituted of angular flattish 

 germinal cells of which no single one can be at first dis- 

 tinguished from the remainder. As growth proceeds one of the 

 cells occupying a central position becomes distinguished from 

 the remaining cells by its greater size, and wedge-like shape. 

 It constitutes the single ovum of the ovary. After it has become 

 prominent it grows rapidly in size, and throws out irregular 

 processes. The germinal vesicle, which for a considerable time 

 remains unaltered, also at length begins to grow ; and the 

 sharply defined germinal spot which it contains after reaching a 

 certain size completely vanishes. After the atrophy of the 

 germinal spot, there appears in the middle of the ovum a 

 number of roundish yolk granules. 



The shape of the ovum becomes more irregular, and chloro- 

 phyll granules, in addition to the yolk granules, make their 

 appearance in it. A fresh germinal spot of circular form also 

 arises in the germinal vesicle. Protoplasmic processes are next 

 thrown out in all directions, giving to the ovum a marvellous 

 amoeboid character. (Fig. 4.) The amoeboid form of the ovum 

 serves no doubt to give it a larger surface for nutrition. Coin- 

 cidently with the assumption of an amoeboid form there appear 

 in the ovum a great number of peculiar bodies. They are 

 vesicles with a thick wall bearing a conical projection into the 

 interior which is filled with fluid. (Fig. 46.) These bodies 

 are formed directly from the protoplasm of the ovum, and are 

 to be compared both morphologically and physiologically with 

 the yolk-spherules of such an ovum as that of the Bird. They 

 are called pseudocells by Kleinenberg, and are found with 

 slightly varying characters in many ova of the Hydrozoa. 



They first appear as small highly refracting granules ; in these a cavity 

 is formed which is at first central but is eventually pushed to one side by the 

 formation of a conical projection from the wall of the vesicle. 



