42 2 



How Anbihils and Plants Reproduce unit viii 



Male 



6 



Opening of oviduct 



Y '' J Blood vessel 

 ^ '^-Kidney 



-Oviduct 



Ureter 

 Cloaca 



Bladder 



Blood vessel 



Kidney 



Spermary 

 (fesfis) 



Ureter v/hich 



carries the 



sperm 



Cloaca-;jr 

 Bladder 



Fig. 369 Reproductive and excretory organs of the male and female frog. When the 

 eggs are full grown, just before being laid, the ovaries are twice as large as indicated 

 in this drawing. How do the male and female organs cojupare in size? 



This is true of many mammals. See Fig- 

 ure 368. Among certain spiders and in- 

 sects on the other hand, the female is the 

 larger and the more ferocious. The 

 smaller male may even end up in the 

 stomach of his mate. Among human be- 

 ings, besides certain differences between 

 the sexes in bodily size and strength, 

 there are differences in the growth of 

 hair on the face and in the voice. 



Secondary sex characters are caused 

 by hormones. Several hormones are 

 formed in the male organs, and others in 

 the female organs. Being hormones, they 

 are carried in the blood to all parts of the 

 body, and their action results in the de- 

 velopment of secondary sex characters. 



The sex organs. Spermaries and ovaries 

 are usually in pairs. Leading from them 

 there are tubes which carry the gametes 

 to the outside. In the male there are nar- 

 row sperm ducts leading to other tubes, 



not directly to the outside; in the female 

 there are wider oviducts (oh'vee-ducts). 

 In the frog the ovaries occupy an unusu- 

 ally large part of the body cavity. If you 

 study Figure 369 and then examine the 

 reproductive organs of a dissected frog, 

 your study of reproduction will be more 

 interesting. See Exercise 2. 



The gametes. The two kinds of gam- 

 etes, eggs and sperms, are clearly different 

 in structure. Egg cells are usually spheri- 

 cal (like a ball). They vary in size. In 

 mammals they are microscopic; in birds 

 thev^ may reach an enormous size. In the 

 hen's Q^^ the \o\V. is a single c^^ cell; it 

 is so lar^e because it contains a ereat 

 amount of food material. All eggs have a 

 nucleus, a cell body of cytoplasm, and a 

 membrane around the cell body. Except 

 for the extraordinarily laroe amount of 

 food material in some eggs, they are 

 much like other cells. 



