288 



ZOOLOGY 



SECT. XIII 



as they pass down the oviducts and are finally stored up in the 

 thin-walled posterior portions of those tubes, which, in the breed- 

 ing season, become immensely dilated and serve as uteri. 



Development.^ — ^The eggs are laid in water in large masses ; 

 each has a black and a white hemisphere, the former always 

 directed upwards, apd is surrounded by a sphere of jelly. The 



egg is telolecithal, the 

 protoplasm being mainly 

 accumulated on the 

 pigmented hemisphere, 

 while the white hemi- 

 sphere is loaded with 

 yolk. During oviposition 

 the male sheds his sper- 

 matic fluid over the 

 eggs, and the sperms 

 make their way through 

 the jelly and impregnate 

 them. In a short time 

 the jelly swells up and 

 becomes thereafter 

 impermeable to the 

 sperms. 



Segmentation begins 

 by a vertical furrow 

 dividing the oosperm into 

 two cells (Fig. 939, A), 

 and soon followed by a 

 second vertical furrow at 

 right angles to the first 

 (B), and then by an 

 equatorial furrow placed 

 nearer the black than 

 the white pole (C). Thus 

 the eight-celled embryo 

 consists of four smaller 

 black cells and four larger 

 white cells. Further 

 divisions take place (D), 

 the black cells dividing rapidly into niicromeres (mi.), the white, 

 more slowly, into megameres (m^.) : as in previous cases, the 

 presence of yolk hinders the process of segmentation. The pig^ 

 mented micromeres (D — F, ?;w'.) give rise to the ectoderm, which is 

 many-layered : the megameres (mg.) contribute to all three layers 

 and are commonly called yolk-cells. During the process of seg- 

 mentation a blastoccele (E, il. ccel.) or segmentation-cavity appears 

 in the upper hemisphere. ' ' 



Fig. 93S.— Rana esculenta. Uiinogcnital organs of 

 the female. iV, kidneys ; Od. oviduct ; Ot, its ccelomic 

 aperture; Ov. left ovary (the right is removed); P, 

 cloacal aperture of oviduct ; S, S\ cloacal apertures 

 of ureters ; Ut, uterine dilatation of oviduct. (From 

 Wiedersheim's Comparatifc Anato^iiy.) 



