34 2 ZOOLOGY. 



layer, supported by connective tissue, forms the essential part 

 of the ovaries and testes, of which there is usually a single pair. 

 The eggs vary in size from T( y T of an inch in mammals 

 to 5 inches (ostrich), or more in some extinct birds. 

 The outlets for the ova and spermatozoa (oviducts 

 and vasa deferentia) are modified portions of the em- 

 bryonic excretory and kidney ducts. Throughout the group 

 there is a close connection between the excretory and the re- 

 productive organs. The oviducts may have special glands for 

 depositing nutritive or protective material about the egg be- 

 fore or after fertilization (as the albumen and shell in egg 

 of birds). Fertilization is external in most fishes and some 

 amphibia, and internal in the higher groups. The uterus is a 

 special portion of the oviduct where early embryonic develop- 

 ment may occur. (See Figs. 202, 203.) 



354. Development. Those eggs which are fertilized out- 

 side develop principally by means of the yolk of the ovum. 

 Those internally fertilized may receive, after impregna- 

 tion, additional materials for the further nourishment of the 

 embryo, as above noticed for reptiles and birds. The fertilized 

 ova may be retained for a longer or shorter time in the ovi- 

 duct or in some modified portion of it (uterus, in mammals) 

 and undergo development there. Where this internal develop- 

 ment is slight (as in birds) the animals are described as 

 oviparous; where it is considerable, as in mammals, and the 

 young are free at birth they are described as viviparous. 



The table on page 343 will give some of the facts concerning the early 

 development of vertebrates. It will be found an excellent exercise to 

 have students verify the data collected in this and the preceding table. 

 The teacher can readily supplement it by a demonstration of figures from 

 more advanced texts. 



355. The Muscular System. We have seen above ( 337) 

 that the internal layer of the mesodermic pockets comes to 

 be united with the digestive tract and furnishes the non-striped 

 muscle fibres of its walls. The external portion, which be- 

 comes associated with the ectoderm, gives rise to the muscles 



