THE OVUM. 29 



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 of more than one cell. 



The development of the ova of the Tubularidae, which has 

 been supposed 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.y. 



PLATYELMINTHES. 



(10) P. Halle z. Contributions a VHistoire naturelle des Turbellaries. Lille, 

 1879. 



(11) S. Max Schultze. Beitrdge z. Naturgeschichte d. Turbellarien. Greifs- 

 wald, 1851. 



(12) C. Th. von Siebold. " Helminthologische Beitrage." Mullet's Archiv, 

 1836. 



(13) C. Th. von Siebold. Lehrbuch d. vergleich. Anat. d. wirbellosen Thiere. 

 Berlin, 1848. 



(14) E. Zeller. " Weitere Beitrage z. Kenntniss d. Polystomen." Zeit.f. wiss. 

 Zool., Bd. xxvu. 1876. 



[Vide also Ed. van Beneden] (No. i). 



This group, under which I include the Trematodes, Cestodes, 



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

 the gonophores of Tubularia mesembryanthemum. Dr Kleinenberg informs me how- 

 ever that the absence of a distinct boundary between the germinal cells and the ovum 

 is not usual. 



