356 



COLLEGE ZOOLOGY 



shady places. Frogs also seem to be stimu- 

 lated by contact as shown by their tendency 

 to crawl under stones and into crevices. 

 Other factors probably have some influence 

 upon this reaction. Temperature and other 

 stimuli modify the responses both to light 

 and to contact. 



Investigators who have studied the be- 

 havior of frogs have come to the conclusion 

 that they are very stupid animals, but it is 

 possible to teach them certain things, and 

 habits once formed are not easily changed. 

 For example, Yerkes found that a frog could 

 learn to follow a path in a labyrinth after 

 about 100 trials. If we consider the power 

 to learn by individual experience as evi- 

 dence of intelligence, we must attribute a 

 primitive sort of intelligence to the frog. 



Life cycle of the frog 



Frogs lay their eggs in water in the early 

 spring. The male clasps the female firmly 

 with his forelegs just behind her forelegs. 

 This is one of the strongest seasonal inborn 

 reflexes (instincts) of the male frog; he will 

 even clasp one's finger when caught during 

 this time of the year. After the male has 

 been carried about by the female for several 

 days, the eggs pass from the uterus out of 

 the cloaca. As the eggs are extruded by the 

 female, they are fertilized by the sperms 

 which the male discharges over them. The 

 male then releases his grip on the female 

 and leaves her. Each female lays several 

 hundred or more eggs, but some of these 

 fail to develop, and others are eaten by ene- 

 mies. 



The jelly which surrounds and protects 

 the eggs soon swells up through absorption 

 of water. The frog egg is well adapted to 

 hatch in the cold pond water of early spring. 

 The upper black surface of the egg absorbs 

 the sun's heat like a black coat. The trans- 

 parent jelly covering holds heat like the glass 

 of a greenhouse. 



Cleavage takes place as indicated in Fig. 

 237. Some of the cells, called macromeres, 

 are large because of a bountiful supply of 



yolk; others, the micromeres, are smaller. A 

 blastula is formed by the development of 

 a cavity, the blastocoel, near the center of 

 the egg. Gastrulation is modified in the frog's 

 egg because of the amount of yolk present. 

 The dark side of the egg gradually grows 

 over the lighter portion untfl only a circu- 

 lar area called the yolk plug is visible. The 

 gastrula contains two germ layers, an outer 

 ectoderm and an inner endoderm. A third 

 layer, the mesoderm, soon appears between 

 the other two and splits into two, an inner 

 splanchnic layer, which forms the support- 

 ing tissue and musculature of the digestive 

 canal, and an outer parietal layer, which 

 forms the connective tissue, muscle, and 

 peritoneum of the body wall. The cavity be- 

 tween these two mesodermal layers is the 

 coelom. 



Soon after gastrulation, the neural groove 

 appears, on either side of which is a neural 

 fold. The neural folds grow together at the 

 top, forming a tube which later develops 

 into the brain and spinal cord of the em- 

 bryo. The neural groove lies along the me- 

 dian dorsal line, and the embr\'0 now length- 

 ens in this direction. The region where the 

 yolk plug was situated lies at the posterior 

 end. On either side near the anterior end 

 two gill arches appear; and in front of each 

 of these a depression arises which unites 

 with its fellow and moves to the ventral 

 surface, becoming the ventral sucker. An 

 invagination soon appears just above the 

 ventral sucker; this is an oval pit which de- 

 velops into the mouth. 



The invagination, which becomes the 

 cloacal opening (anus) appears beneath the 

 tail at the posterior end. On either side 

 above the mouth, a thickening of the ecto- 

 derm represents the beginning of the eye, 

 and just above the gills appear the invagina- 

 tions which form the vesicles of the inner 

 ears. The markings of the muscle segments 

 show through the skin along the sides of 

 the body and tail. 



The embryo moves about within the egg 

 membranes partly by means of cilia, but 

 these soon disappear after hatching (Fig. 



