212 THE FROG CHAP. 



Notice the openings into the bladder and large intestine respectively, 

 and also : — 



In the male, the apertures of the ureters (urinogenital ducts), situated 

 on two small papillje, lying close to one another on the dorsal side of 

 the cloaca : insert a bristle into the ureter. Sketch. 



In the female, the two small apertures of the ureters, and just in front 

 of these, the two large apertures of the oviducts, all situated on the 

 dorsal wall of the cloaca. Insert bristles into the ureter and oviduct. 

 Sketch. 



Impregnation and Development. 



In the early spring (end of February or beginning of March), look 

 about in ponds and ditches for frogs which have taken to the water for 

 the purpose of laying eggs; This process can be watched more con- 

 veniently by catching a few frogs, male and female, and putting them 

 in a large vessel of water or aquarium. If you have been unsuccessful 

 in procuring frogs or frogs' spawn, toads will do equally well : their 

 eggs are laid a few weeks later, and are arranged, each surrounded by 

 its gelatinous envelope, not in clumps, like those of the frog, but in a 

 string, like the beads on a necklace. 



For purposes of observation, the spawn is best kept in a glass vessel, 

 together with some water-weeds. Put only a small quantity of spawn 

 in one vessel; if the water begins to get foul, change it at once. 

 Examine the eggs every day with a magnifying-glass as development 

 proceeds, and keep some of the tadpoles alive until metamorphosis 

 takes place in June. 



If you wish to see some of the stages in the development of the frog 

 during other times of the year, you must obtain some preserved eggs 

 and embryos, and, if possible, you should also examine the series of wax 

 models, made on an enlarged scale, which are to be seen in most 

 zoological museums. 



The following are some of the more important things to be noticed. 

 (Sketch a series of stages) : — 

 Before hatching. 



I. The unsegmented oosperm, black above, and white below. 



II. Early stages in segmentation, during which the individual cells 

 can be made out by means of a lens. (Fig. 64 A-F). 



III. Later stages in segmentation, during which the individual cells 

 cannot be distinguished without the aid of a microscope. Note the 

 gradual enclosure of the white by the black hemisphere, until only a small 



