54 CHORDATA. 



In Amphioxus and the Craniata the ova are developed as in 

 the Chaetopoda, Gephyrea, etc., from specialized germinal cells 

 of the peritoneal epithelium. 



In Amphioxus the germinal epithelium which constitutes the 

 essential part of the ovary is divided into a number of distinct 

 segments : in the Craniata no such division is observable. 



In young examples of Amphioxus the generative organs are 

 in an indifferent condition, and the two sexes cannot be dis- 

 tinguished. They form isolated horse-shoe shaped masses of 

 cells, which occupy a position at the base of the myotomes, in 

 the intervals between the successive segments ; and extend from 

 the hinder end of the respiratory sack to the abdominal pore. 

 They are situated in the proper body cavity, and are surrounded 

 by the peritoneal membrane. Each generative mass is at first 

 solid, and is formed of an outer layer of more flattened cells and 

 an inner mass of large rounded or polygonal cells. In its interior 

 there appears at a somewhat later period a central cavity. After 

 the cavity has appeared the sexes can be distinguished by the 

 different behaviour of the cells. 



In all the Craniata, the ovary forms a paired ridge (unless 

 single by abortion or fusion) attached by a mesentery to the 

 dorsal wall of a more or less extended region of the abdominal 

 cavity. This ridge is at first identical in the two sexes, and 

 arises at an early period of embryonic life. It is essentially 

 formed of a thickening of the peritoneal epithelium, and in 

 Osseous Fish, Ganoids (?) and Amphibia the ovary remains 

 during embryonic life nearly in this condition, though a small 

 prominence of the adjacent stroma also becomes formed. In 

 other Craniata the ridge, though at first in this condition, very 

 soon becomes much more prominent, and is formed of a central 

 core of stroma enclosed in the germinal epithelium (fig. 18). 



The thickened germinal epithelium gives rise (in the case of 

 the female^ to the ova and the follicular epithelium. Whether 

 the genital ridge is provided with a core of stroma or no, the 

 germinal epithelium is always in contact on one side with the 

 stroma, from which it is at first separated by a well-marked 

 boundary line ; but after a certain time there appear numerous 

 vascular ingrowths from the stroma, which penetrate through all 

 parts of the germinal epithelium, and break it up into a sponge- 



