THE OVUM. 35 



and a nucleated peripheral layer of each cell becomes separated from 

 the central part, which also contains nuclei. This latter part next 

 divides into numerous cells, of which one eventually forms the ovum, 

 and the remainder constitute a mass of cells adjoining it as in 

 Bonellia (fig. 16). This mass of cells eventually disappears, and is 

 probably employed in the nutrition of the ovum. 



The ovaries of the Leech appear to belong to the tubular type in 

 that the ova are not formed from part of the epithelium lining the 

 body cavity; but if, as seems probable, the true affinities of the 

 Leeches are with the Chastopoda, the investment of the ovaries must 

 be of a secondary nature. It should be noted that the ova are not, as 

 in the ordinary tubular ovary, developed from the epithelium lining 

 the ovarian tube. 



GEPHYREA. 



(40) Keferstein u. Ehlers. Zoologische Beitrage. Leipzig, 1861. 



(41) C. Semper. Holothurien, 1868, p. 145. 



(42) J. W. Spengel. "Beitrage z. Kenntniss d. Gephyreen." Beitrage a. d. 

 zool. Station z. Neapel, Vol. i. 1879. 



(43) J. W. Spengel. " Anatomische Mittheilungen iib. Gephyreen." Tagebl. d. 

 Naturf. Vers. Munchen, 1877. 



In the Gephyrea, as in the ChaBtopoda, the ova are developed from 

 the lining cells of the peritoneum and frequently from the cells 

 surrounding parts of the vascular system (Bonellia, Thalassema). In 

 many cases (Sipunculus, Phascolosoma, Echiurus) the main growth 

 of the ovum takes place after it has been dehisced into the body 

 cavity. 



In Sipunculus the ova in the body cavity are surrounded by a 

 follicle which is thrown off before they become ripe. 



Brandt denies the existence of this follicle or rather its cellular nature. 

 Spengel's (43) observations are conclusive in favour of the correctness of 

 the original interpretation of Keferstein and Ehlers. The follicles would 

 seem to be formed after the ova have become free. In Phascolosoma there 

 is no follicle (Semper, Spengel). 



In both Phascolosoma and Sipunculus a vitelline membrane with 

 radial pores zona radiata is formed, and in Phascolosoma the ex- 

 ternal part of this is separated off as a structureless vitelline membrane. 

 The formation of both these membranes from the protoplasm of the 

 ovum is rendered certain in the latter case by the absence of a 

 follicular epithelium. 



Some interesting observations on the growth and origin of the 

 ovum in Bonellia have been made by Spengel. 



The ova originate from certain cells (germinal cells) in the 

 peritoneal investment of the ventral vessel, overlying the nervous 

 cord. These cells, which are well marked off from the surrounding 

 flattened peritoneal elements, increase in number by division, and 

 form small masses surrounded by a follicle of peritoneal cells, and 



32 



