XII TRACTICAL DIRECTIONS 211 



seminal tubes or crypts which open into it. Under the high power, 

 observe the epithehal cells {germinal epitlieliuin), lining the tubes and 

 their subdivision into smaller cells, which eventually give rise to the 

 sperms, the tails of which project freely into the cavities of the tubes. 

 Sketch. 



II. Female Organs. 



1. Notice again {a) the ovaries, varying in size and appearance ac- 

 cording to the time of year, and each suspended by a fold of peritoneum : 

 they are studded all over with ovisacs, each of which contains an egg or 

 (nnitn, pigmented when ripe ; and (b) the oviducts. (Fig. 4.) Trace the 

 convoluted and glandular middle portion of the oviduct forwards, and 

 make out the anterior thin-walled portion, running parallel to the gullet 

 and opening into the coelome by a small aperture at the base of the 

 lung ; then trace the middle portion backwards, and notice the thin- 

 walled, dilated, posterior portion, which opens into the cloaca. Sketch 

 after examining the cloaca (see below). 



If a female frog is examined in the spring, just before the eggs are 

 laid, the ovaries will be seen to be reduced in size, and the posterior 

 portions of the oviducts filled with eggs, each surrounded with a gela- 

 tinous coat secreted by the middle portion of the oviduct. 



2. Examine under the microscope a section of the ovary, prepared as 

 before (p. 136), first with the low, and then with the high power 

 (Fig. 63). Note the epithelial cells (including \}At germinal epithelium] 

 forming the walls of the ovary, and the ovisacs in different stages of 

 development. Each ovisac contains an oznim, surrounded by follicle- 

 cells. In the ova, note the protoplasm and its contained yolk-granules, 

 the vitelline membrane, the nucleus, and the nucleoli. Sketch. 



Cloaca, 



Carefully cut through the pelvic symphysis in the middle line with a 

 scalpel and press apart the two innominate bones, so as to expose the 

 ventral surface of the cloaca. Inflate from the vent, and note that the 

 large intestine is continuous with the cloaca and that the urinary 

 bladder opens into it on the ventral side. 



The entire urinogenital apparatus, together with the cloaca, may 

 now be removed from the body, first cutting through the skin round the 

 vent. Pin down under water, insert the small scissors into the vent, 

 and slit open the cloaca very slightly to one side of the middle line, 

 so as not to injure the connection between the bladder and cloaca. 



P 2 



