AMPHIBIA 



225 



convoluted oviduct or Miillerian duct. The darkly pigmented 

 eggs are coated with a thin layer of albumen by the glandular 

 secretion of the oviduct. At the lower part of its course the 

 oviduct widens, forming a cavity in which the eggs may remain 

 before being deposited in the water. 



During the breeding season, in early spring, the male grasps 

 the female, the anterior limbs with their swollen pads being 

 firmly flexed around the pectoral region of the female. The 

 eggs and the sperms are discharged simultaneously, and 

 fertilisation takes place in 

 the water. 



Development. The eggs 

 shortly after extrusion 

 become distended by the 

 absorption of water in the 

 albumen layer, and the 

 whole of the eggs laid 

 form a floating mass at 

 the surface. Each egg is 

 surrounded by a vitelline 

 membrane around which 

 the albumen layer forms 

 a thick coat of a pro- 

 tective nature. The egg 

 is darkly pigmented, and 

 the warmth obtained from 



the rays of the sun is retained in contact with the albumen, 

 better at least than it would be in contact with water. 



The first polar body is liberated when the egg becomes ripe 

 in the ovary, and the second, when the egg is extruded and 

 when the sperm gains entrance, as a preliminary to fertilisation. 



Segmentation is holoblastic, and the first two furrows are 

 mutually at right angles through the polar field. The egg is 

 thus orientated as in Amphioxus. The third cleavage is at 

 right angles to both the others, and separates the more animal 

 from the more vegetable region. Thereafter the cells at the 

 animal pole divide the more rapidly, and a blastula is formed 

 consisting of small pigmented cells at the animal pole and of 

 larger and fewer white cells at the opposite pole, and enclosing 



FIG. 114. Segmentation of egg and begin- 

 ning of delamination. After Marshall. 



