264 



PRACTICAL ZOOLOGY 



The eggs arise in the two ovaries of the female (Fig. 149, B, 5), 

 make their way into a pair of tubes, the oviducts (7), and from 

 there into the distensible uterus. Here they remain until they 

 are ready to be laid, when they pass out through the anal 

 opening. 



EGG LAYING. The eggs of the frog are laid in water in the 

 spring. As soon as they emerge from the female they are fer- 

 tilized by spermatozoa poured over them by the male. Then 



FIG. 151. Stages in the early development of the frog's egg. (From Ecker.) 



the jelly which surrounds them swells in the water and effec- 

 tively protects them from injury. 



EMBRYOLOGY. The development of the egg which takes 

 place within this coat of jelly is known as embryology and the 

 partially developed egg is an embryo. One of the most remark- 

 able of all natural phenomena is the development of a complex 

 adult animal from an apparently simple egg. To understand 

 how this takes place we must study the changes that go on 

 within the egg. 



When laid, the egg is a single cell. The spermatozoon is also 

 a single cell. These two cells unite in fertilization and become 

 fused into one. The nucleus of the egg and the nucleus of the 

 spermatozoon approach each other and also fuse into a single 

 nucleus. 



The single cell thus formed, the fertilized egg, now proceeds to 

 divide into two cells. Inside of the egg the single nucleus di- 

 vides, by a process called mitosis, into two. During mitosis (Fig. 



