THE OVUM. 23 



The shape of the ovum becomes more irregular, and chlorophyll 

 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. Coincidently 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. 4 B.) These bodies are formed directly 

 from the protoplasm of the ovum, and are to be compared both morpho- 

 logically 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. 



After the growth of the ovum is completed the amoeboid processes 

 gradually withdraw themselves, and the ovum assumes a spherical 

 form ; still however continuing to be invested by the remaining cells 

 of the ovary. It is important to notice that the egg of Hydra retains 

 throughout its whole development the characters of a single cell, and 

 that the pseudocells and other structures which make their appearance 

 in it are not derived from without, and supply not the slightest 

 ground for regarding the ovum as a structure compounded ot more than 

 one cell. 



The development of the ova of the Tubularidae, which has been 

 suppesed by many investigators to present very special peculiarities, 

 takes place on essentially the same type as that of Hydra, but 

 the germinal vesicle remains permanently very small and difficult to 

 observe. The mode of nutrition of the ovum may be very instructively 

 studied in this type. The process is one of actual feeding, much as 

 an Amoeba might feed on other organisms. Adjoining one of the large 

 ova of the ovary there may be seen a number of small germinal cells. 

 (Fig. 3). The boundary between these cells and the ovum is indistinct. 

 Just beyond the edge of the ovum the small cells have begun to 

 undergo retrogressive changes ; while at a little distance from the 

 ovum they are quite normal (g.c) l . 



1 The above description of the ova of the Tubularidas is founded on sections of 

 the gonophores of Tubularia mesembryanthenmm. Dr Kleineuberg informs rue however 

 that the absence of a distinct boundary between the germinal cells and the ovum is not 

 usual. 



