REPRODUCTION AND DEVELOPMENT IN CHORDATES 



mammals the ostium practically surrounds the ovary (Fig. 5.2). When 

 ovulation, or rupture of the follicles, occurs in the frog, the eggs are dis- 

 charged from the ovary and drop into the coelom, where they are carried 

 through the ostia of the oviducts by the beating of cilia. In higher forms 

 the eggs pass directly into the ostia as ovulation occurs and are present in 

 the coelom only under atypical conditions. 



Attention may be called here to the several meanings which the word egg 

 has. We speak of birds' and reptiles' eggs, which most often have been 

 fertilized and are in the process of development, and which contain albuminous 

 envelopes in addition to the female germ cell (Fig. SAB). We refer to ovarian 

 eggs when we mean the growing oocytes, and to tubal eggs when we mean 

 recently ovulated female germ cells which may be either oocytes or ova (p. 

 139) or may be fertilized and in the early stages of development. The student 

 should not be confused by the several uses of this word, which are so widely 

 accepted that it would be impossible to change them. Wherever clarity 

 demands it, a more restricted term will be used; in other cases, the meaning 

 will be clear from the context. 



The frog has a relatively unspecialized oviduct. Jelly is secreted by the 

 cells lining the long coiled portion, and the eggs may be stored temporarily 

 in the expanded ovisac which opens into the cloaca, through which the eggs 

 pass to the outside by way of the anus. The albumen, or so-called white, of 

 a reptile's or bird's egg is secreted by a specialized region of the oviduct; the 

 egg envelopes and shell are secreted by different regions. Animals 

 that lay eggs which are well supplied with food and in which the young 

 develop outside the body are known as oviparous. Among the fishes and 

 reptiles some forms retain the fertilized eggs in a uterine portion of the 

 oviducts during development. In such species the young are nourished by the 

 food stored in the egg, and these forms are referred to as ovovlvlparous. 

 A few of the most primitive mammals, such as Ormlhorhynchus, the duckbill 

 (see Fig. 18.30^4, p. 581), lay eggs and have paired oviducts like those of 



Animal pole 



Nucleus 



Fig. 5.3. Germ cells of the frog, show- 

 ing difference in size. A, mature 

 spermatozoa: xl070. B, an oocyte, 

 with its follicle, at the end of the 

 growth period: x30; drawn in sec- 

 tion to show the unequal distribu- 

 tion of yolk in the animal and vegetal 

 hemispheres and the location of the 

 nucleus in the animal hemisphere; 

 the arrow indicates the egg axis. 



