THE OVUM. 43 



cells, but are believed to originate spontaneously in the vitellus. 

 Finally by Semper they are believed not to be cells, but to be 

 amoeboid protoplasmic bodies which are pressed out from the vitellus 

 under the stimulus of the sea-water or otherwise. 



They do not according to this author naturally appear till the ovum 

 is quite ripe, though they can be artificially produced at an earlier period 

 by the action of reagents or sea-water. When produced in the natural 

 course of things the vitellus undergoes a contraction. They are without 

 any apparent function, and play no part in the embryonic development. 

 Semper's results are very peculiar, but owing to the careful study which 

 his paper displays they no doubt deserve attention. Further investigations 

 are however very desirable. Kowalevsky from his researches on Pyrosoma 

 (No. 59) adheres to his first opinion, though he abandons the view that 

 these cells are connected with the formation of the test. 



In the passage of the egg through the oviduct the vacuolated 

 follicle cells grow out into very peculiar long processes or villi. In 

 Ascidia canina these processes become as long as the whole diameter 

 of the vitellus (Kupffer, No. 60). 



In Amphioxus and the Craniata the ova are developed as in the 

 Chsetopoda, 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 distinguished. 

 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 respira- 

 tory 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 epi- 

 thelium (fig. 18). 



