426 DEVELOPMENT OF FROG AND OTHER VERTEBRATA 



the early stages, are almost completely degenerate in the adult 

 animal, leaving the left ovary and oviduct as the functional parts. 

 The spermatozoa that pass to the cloaca of the female during 

 sexual union enter the oviduct and make their way to its anterior 

 end, where they may be found when the egg leaves the ova^y^ 



€nlr 



Of 



^3 



"l' V"clf ^'"^' 



Fig. 223.- 



-Urinogenital organs of the pigeon, Colutnba lii-ia. 

 system. Right, the female system. 



Left, the male 



adr, adrenal body; cl.^, urodseum, or ventral portion of cloaca; cl.^, proctodaeum, or 

 dorsal portion of cloaca; k, kidney; l.od, left oviduct; l.od', opening of left oviduct into 

 cloaca; l.od", left mesovarium, or mesentery of ovary; l.od'", left Fallopian tube; od, 

 right oviduct; or, ovary; r.od', opening of right oviduct into cloaca; ts, testis; ur, ureter; 

 ur', opening of ureter into cloaca; v.d., vas deferens; v.d.', opening of vas deferens into 

 cloaca; r.s., seminal vesicle. (From Parker and Haswell, "Textbook of Zoology," copy- 

 right, 1921, by Macmillan and Co., Ltd., reprinted by permission.) 



The ovary of a laying hen (Fig. 224 A) contains ova in all stages, 

 from very small ones, no larger than those in the ovary of a frog, 

 to others that are about readj^ to be laid and are, therefore, the 

 full size of the " yolk " of the hen's egg. The ovum of the fowl 

 (Fig. 224 B) may, therefore, be compared with a frog's egg so 

 greatly enlarged by additions of yolk that the active protoplasm 

 has become restricted to a small area at the animal pole, composed 

 of a disc-shaped mass of cytoplasm containing the egg nucleus 

 and called the germinal disc or blastodisc. As a matter of fact, 

 the eggs of the fowl arise by growth from the smaller ova, as may 

 be seen even by superficial examination. At the time of o\iila- 



