CHAPTER XVIII. DEVELOPMENT OF THE FROG 



1. Maturation and Fertilization. The ripe ova of the 

 frog are about yr f an inc h i n diameter, or five thousand 

 times as large as those of Amphioxus. This relatively large 

 size is due to the enormously greater amount of yolk con- 

 tained within the protoplasm, especially of the vegetative 

 hemisphere. The nucleus (germinal vesicle) lies near the 

 animal pole, and the protoplasm around it contains little 

 or no yolk, but it contains much black pigment. Thanks 

 to this black colour, the animal hemisphere is readily dis- 

 tinguished from the vegetative. Such an ovum as this, 

 having its yolk concentrated towards one pole, is said to be 

 telolecithal. 



About the time that maturation is beginning, the ova 

 escape in large numbers from their follicles and fall into 

 the coelom, each surrounded only by its vitelline membrane. 

 They travel forwards (how, is not fully understood) and 

 enter one or other of the internal apertures of the oviducts. 

 As each ovum passes down - the oviduct, it becomes sur- 

 rounded by a layer of albuminous substance secreted by the 

 oviduct walls. During this time the process of maturation 

 begins, and when the eggs are laid one polar body has been 

 formed. 



As the eggs are laid they are fertilized by the male, who 

 lias been waiting, attached to the female, for days before- 

 hand. It is a curious point that the second polar body is 

 not actually extruded until after the male pronucleus has 

 entered the ovum. 



The eggs are laid in ponds or backwaters of streams 

 about March. On contact with the water the albuminous 

 covering swells up and serves both to float the eggs and to 

 protect them from the many enemies that would otherwise 

 soon devour them in their exposed situation. 



2. Segmentation begins after the same fashion as in 



228 



