TKANSVEilSE SECTIONS. 103 



emhryos ; cut the gill across ; remove some of the embryos, 

 mount them on a slide in water, and examine them with a 

 microscope. 



III. EXAMINATION OF TEANSYEESE SECTIONS. 



Several points in the anatomy of the mussel, and notably 

 the relations of the gills, kidneys, and heart are best studied 

 by means of a series of transverse sections. 



For this purpose take a good-sized specimen, and put it 

 into J per cent, chromic acid, wedging the valves slightly open 

 so that the acid may have free access to the branchial cavity. 

 After a couple of days transfer it to spirit, and keep it till 

 wanted. 



To make the sections, remove both valves of the shell care- 

 fully ; place the animal on a board, and ciU it transversely 

 with a razor into a series of slices about a quarter of an inch 

 thick ; arrange the sections in order in a dissecting dish 

 under water ; examine and draw them. 



The most instructive sections are those passing (1) through 

 the renal and reproductive apertures ; (2) through the middle 

 of the ventricle ; (3) through the posterior adductor. The 

 chief features of these sections are as follows. 



A. Transverse Section through the Renal and Reproductive 

 Apertures. 



The section passes through the widest part of the body, a 

 little way behind the umbo. 



1. The mantle-folds arise very close to the dorsal surface, 



one on each side of the pericardial cavity. The 

 folded margin which secretes the two outer layers 

 of the shell should be noticed. 



2. The visceral mass is large and laterally compressed, 



and lies in the middle of the section between the two 

 mantle-lobes. The chief part of its substance is 

 made up of the lobes of the ovary, or testis. The 



