CARMINE-STAINED PECTINATELLA 85 



a hair, they contract slowly, it is evident that they are partly narcotized. One 

 must be exceedingly careful not to disturb them further for at least 10 min- 

 utes, for if they contract in a narcotized condition they will not expand again. 

 The right stage for killing has arrived when no amount of shoving with a 

 hair will persuade the specimens to contract and an examination under a 

 binocular microscope shows that the ciliary action on the lophophore has not 

 stopped. A rubber tube is used to siphon carefully from the finger bowl enough 

 water so that the remaining layer just covers the specimens. Then the finger 

 bowl is filled with 4 per cent formaldehyde, covered, and put to one side. 



One must be very careful to distinguish between a "killing" agent, as formal- 

 dehyde, and "hardening" and "fixing" agents. In the present instance it is quite 

 unnecessary, since a stain containing in itself an adequate mordant is to be 

 used, to employ any fixative which will combine with the proteins of the 

 specimens, but it is necessary to harden them, in order that they may with- 

 stand the treatment to which they will be subjected in staining and dehydra- 

 tion. Four per cent formaldehyde hardens very slowly, and it is suggested that 

 next the specimens be passed to alcohol for the hardening process. 



It is desirable, however, that they be flattened, before hardening, into the 

 shape that they will be required to assume after mounting. It is to be pre- 

 sumed that the purpose of making a microscope slide is to study the object 

 which has been mounted; the depth of focus of microscope lenses is so slight 

 that only relatively thin objects can be studied. It is extraordinary how fre- 

 quently this simple principle is overlooked or how frequently people endeavor 

 to flatten the object after it has been mounted in balsam and is almost inva- 

 riably so brittle that it will break up during the flattening process. Five min- 

 utes' work in arranging the parts before hardening makes all the difference 

 between a first-class and a second-class mount. To arrange and flatten the 

 objects for hardening, the 4 per cent formaldehyde is replaced with water— a 

 matter of convenience— and then the first specimen to be treated is selected. 

 This specimen is removed to a finger bowl of clean distilled water where it is 

 examined thoroughly to make sure it has no adherent dirt. The object is flat- 

 tened by hardening it between two slides, but it will be obvious that if it is 

 just pressed between two slides it will be squashed rather than flattened. Any- 

 thing may be used to hold the two slides apart, though in the present instance 

 a very thick No. 3 or two No. 2 coverslips would give about the right sepa- 

 ration. Therefore, at about an inch on each side of the center of a glass slide 

 is placed a thick No. 3 coverslip, which may be held in place by the capillary 

 attraction of a drop of water. The specimen is picked up from the water with 

 a large eye-dropper type pipette and placed in a large volume of water on the 



