CELL-MEMBRANES. 61 



of the protoplasm itself. In old cells the cell-substance may 

 entirely disappear. The nucleus varies in form, size, internal 

 structure, and physical consistency. It may, apparently be want- 

 ing. The membrane is often wanting ; when present it varies in 

 chemical composition, thickness, and form, and is often compli- 

 cated by the presence of various kinds of prominences, depres- 

 sions, or perforations, variously arranged. In tissues, the indi- 

 vidual membranes are either distinct or more or less fused to- 

 gether, and various lifeless matters may be deposited within their 

 substance. 



PEACTICAL STUDY. 



Since the student has already examined a number of different cells, it 

 is here only desirable to study a few additional forms to illustrate certain 

 special points; and if the material cannot be procured the practical study 

 may be omitted, wholly or in part. 



1. Examine the small transparent ovum of some animal, either 

 fresh or in the prepared specimen. The most favorable are those 

 of starfish or sea-urchins, but those of earthworms, leeches, snails, 

 or fresh-water mussels will answer. In the lack of these, examine 

 prepared sections of the ovary of a mammal (cat, rabbit, etc.), 

 and observe the ova. Fresh specimens should be treated with 

 2$ acetic acid. 



2. Observe some of the stages in the cleavage of the ovum. 

 Specimens may usually be obtained by keeping various kinds of 

 fresh-water snails (Pliysa, Planorbis, etc.) in aquaria, where they 

 readily lay eggs. It is difficult to follow more than the first two 

 or three divisions. In spring the process may be easily studied 

 in the ova of toads, frogs, or salamanders. 



3. Spread on a clean cover-glass a thin film of newt's or frog's 

 blood, and dry perfectly in the air. Pour on a strong solution of 

 haematoxylin (Grenadier's or Kleinenberg's), and leave for 10-15 

 minutes without allowing it to dry. Wash off the dye by a gen- 

 tle stream of water from a faucet or poured from a beaker, add a 

 very dilute aqueous solution of eosin ; leave for a minute, wash 

 again thoroughly, and then dry the cover. Add a drop of dis- 



