184 AMPHIOXUS 



the atrial folds : these cavities are readily recognis- 

 able while the ovaries are young, but become almost 

 obliterated when the ova ripen. 



The ova, when mature, are discharged into the 

 atrial cavity by dehiscence of the proper wall of the 

 ovary and of the atrial membrane. The discharged 

 ova, together with the projecting ovaries, form a 

 bulky mass, which causes great distension of the 

 atrial cavity and distortion of the pharynx and other 

 organs. 



The ova, which measure ^^ inch in diameter, 

 ultimately pass through the gill -slits into the 

 pharynx, and escape through the mouth ; or possibly 

 through the hyoidean apertures. 



2. The male. 



The testes are similar in form and position to the 

 ovaries ; and the spermatozoa, when ripe, are dis- 

 charged like the ova into the atrial cavity, from which 

 they escape by the atrial pore. 



III. EXAMINATION OF TEANSVEESE SECTIONS. 



Mawy points in the anatomy of Am/phioxus are determined 

 far more readily by an examination of transverse sections of 

 the whole animal than by dissection. For this ]pv/rpose an 

 entire specimen should be cut by means of a microtome, and 

 the sections mounted on slides in their proper sequence. It 

 is only necessary to examine in detail a few of the more 

 typical ones. 



A. Transverse Section passing througli the Buccal Cavity. 



1. Greneral characters. The dorsal surface is indicated by 

 the laterally compressed dorsal fin : the ventral 

 surface by the mouth-opening, which is bordered at 

 the sides by the thickened lips ; within the buccal 

 cavity are usually seen sections of the tentacles. 

 About the middle of the section is the notochord, 

 and above this is the spinal cord. At the sides are 



