xii PRACTICAL DIRECTIONS 575 



must be superficial or peripheral (p. 383), and in the 

 meroblastic telolecithal ovum discoid, or restricted to the 

 small germinal disc at its upper pole (Figs. 128 and 149). 

 In the case of holoblastic ova the segmenting cells or 

 blastomeres may be equal, or nearly equal, in size (e.g., 

 lancelet, rabbit) ; or if the yolk is present in greater 

 quantities towards the lower pole, unequal (e.g., earth- 

 worm, frog). 



The influence of the food-yolk in modifying the early 

 processes of development is thus evidently very great, 

 and in order to understand these processes in their 

 simplest form it is necessary to select for our study a 

 microlecithal holoblastic egg, such as that of the 

 lancelet. 



Before reading the next chapter, however, you should 

 once more glance at the account of the development of the 

 tadpole given on pp. 198-210, in order to refresh your 

 memory of some of the more important facts and of the 

 terms used. 



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 this, 

 a good deal may be made out as regards the chromatin and 

 the changes which the chromosomes pass through in the 

 process of mitosis by the following simple method. 



Obtain a young gilled-larva or tadpole of the common newt 

 (p. 218) 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 or of alum-carmine (p. 136). 



Strip off small pieces of the skin of the stained prepara- 

 tion, and after putting them through weak and strong 

 alcohol, transfer to absolute alcohol, then to turpentine, 

 xylol, or oil of cloves, and mount in Canada balsam (p. 139). 

 Examine, comparing Figs. 146 and 147 and pp. 558-563, 

 and sketch as many different stages as possible. 



