PRACTICAL DIRPXTIONS 585 



PRACTICAL DIRECTIONS. 



A. Special methods are required to follow out the details of the 

 structure and division of nuclei, but if you have not the opportunity 

 of examining preparations illustrating these, a good deal may be made 

 out as regards the chromatin and the behaviour of the chromosomes 

 in the process of mitosis by the following simple method. 



Obtain a young, gilled-larva or tadpole of the common newt (p. 218), 

 kill, and place it in corrosive sublimate for about half an hour : wash 

 thoroughly in water and transfer first into weak, and then into strong 

 alcohol (p. 137). Stain entire, either with a solution of hcematoxylin, 

 which can be bought ready prepared, or alum-carmine, which you can 

 prepare yourself by dissolving 200 grams of ammonia-alum in water, 

 boihng the solution and adding carmine to excess, filtering, and diluting 

 with three or four times its bulk of water ; a drop or two of carbolic acid 

 should be added to the solution. 



Strip off small pieces of the skin of the stained preparation, and after 

 putting them through weak and strong alcohol, transfer to absolute 

 alcohol, then to turpentine or oil of cloves, and mount in Canada balsam 

 (p. 139). E.xamine, comparing Fig. 136 and pp. 541-545, and sketch 

 as many different stages as possible. 



B. Observation of the details concerned in the process of maturation 

 and fertilization of the ovum is too difficult for the beginner, but polar 

 cells may easily be seen in the living, freshly-laid eggs of one of the 

 common pond-snails [e.g. Linuicpus stagnalls), in which some of the 

 stages of segmentation can also be observed. Keep some of these snails 

 in a glass vessel with water weeds, and notice that the eggs, when laid, 

 are enclosed, a number together, in a common gelatinous envelope. 

 Separate the eggs from one another with needle, and examine in water 

 under the microscope. Note that the small egg is surrounded by fluid 

 enclosed in a relatively large egg-case, and observe the minute polar 

 cells at the periphery of the ovum. 



C. A series of models of the development of Amphioxtis, to be found 

 in most museums, should be carefully examined (compare Figs 141- 

 144). 



■ D. In order to follow out the development of the chief organs in ,i 



