476 E. A. MINOHIN. 



allowed to act too long. The collar-cells become detached 

 from their position, and are found loose in the tubes. When 

 this process has gone too far the elements of the dermal layer 

 may become displaced also. For surface views slight macera- 

 tion is not always harmful, since it makes it easier to remove 

 the collar-cells, but it is certainly to be avoided in material 

 for sections. Hence care should be taken not to let the osmic 

 acid act any longer than is absolutely necessary for the fixa- 

 tion of the cells. For studying the dermal layer in surface 

 views it is generally desirable to remove the collar-cells as far 

 as possible. This can be effected by brushing the gastral 

 surface of the body-wall gently with a fine paint-brush when 

 the specimens are in glycerine. In the species without quad- 

 riradiate spicules the collar-cells can be removed easily in this 

 way without damaging any other elements. In the species 

 with gastral rays to the spicules it is not so easy to clear away 

 all the collar-cells, and the cells on the gastral rays are also 

 brushed off in the process. 



As a medium for mounting the pieces of the body-wall for 

 surface views glycerine is greatly to be preferred to Canada 

 balsam ; cells can be focussed much more clearly at different 

 depths, and the refraction of the spicules interferes less with 

 the distinctness of the vision. Glycerine has, however, the 

 disadvantage that it acts slowly upon the spicules. After a 

 time they become corroded, and finally dissolved. The time 

 this takes varies greatly in different preparations ; I have some 

 two years old which are only slightly corroded, while others 

 go quicker. As a rule the corrosion does not become marked 

 for at least six months, so that plenty of time is allowed for 

 studying the preparations, though it should not be deferred 

 too long. As a matter of fact this corrosion takes place also, 

 as a rule, in preparations mounted in Canada balsam, though 

 much more slowly. It is very little use, I have found, trying 

 to isolate the elements of the dermal layer. Good pores and 

 epithelial cells may sometimes be obtained in this way, but the 

 spicule cells almost invariably become separated from their 

 spicules. 



