576 PRACTICAL DIRECTIONS CHAP. 11 



B. For the structure of ova, examine again your sections 

 of the frog's and rabbit's ovary (see pp. 211 and 549). Also 

 obtain an unincubated fowl's egg, place it in a small dish or 

 tin about 2i inches deep : mix some plaster of Paris in water 

 and pour it round the egg (first greasing the latter to prevent 

 it adhering to the plaster) so as to cover the lower half, and 

 then let it set. Fill the dish with normal salt-solution, and 

 with the forceps tap the portion of the shell lying uppermost 

 so as to fracture it, when it can be removed bit by bit. Note : 

 a, the accessory parts of the egg developed from the oviduct, 

 vi/., the porous shell, the double shell-membrane the layers 

 of which are closely applied together except at the thicker 

 end of the egg (see Fig. 149), and the white or albumen, with 

 the twisted chalazce at either end ; and, b, the essential part 

 (the yolk) with its vitelline membrane and circular blastoderm, 

 which always floats uppermost. 



Living sperms have already been examined (see, e.g., 

 pp. 210 and 548, and compare Fig. 148) ; the head, middle 

 piece, and tail should be noted. 



C. Observation of the details concerned in the process of 

 maturation and fertilisation of the ovum is too difficult for 

 the beginner 1 but polar bodies may be seen in the living, 

 freshly-laid eggs of one of the common pond-snails (e.g., 

 Limncea stagnulis], in which also some of the stages of seg- 

 mentation can easily 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 needles, 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 bodies at the periphery of the ovum. 



1 The swarming of sperms round the ova previously to fertilisa- 

 tion can easily be observed in the common olive-brown seaweeds 

 (Fucus), by getting fresh material and examining it in sea- water. 



