THE DEVELOPMENT OF VERTEBRATA. 297 



The more reasonable conclusion is that the mechanical 

 stimulation caused by fighting set up the growth and 

 branching of antlers, and that spontaneous variation or 

 mutations gave rise to a number of types in different 

 species. There is everywhere spontaneous tendency to 

 the formation of species, just as there is to the formation 

 of varieties in domesticated animals. Similarly in flat- 

 fishes, all the species show the chief modifications adapted 

 to the exceptional position of the fish, and again, differ- 

 ences of the jaws and teeth are adapted in different flat- 

 fishes to the different kinds of food. But it is impossible 

 to perceive any relation between the character of the 

 scales and the colour markings by which the species are 

 distinguished and differences in the mode of life. Some 

 flat-fishes like the sole have spiny scales, others small 

 smooth scales like the plaice, and others like the flounder 

 and turbot no true scales at all. 



Questions on Vertebrate Development. 



1. Describe the changes in the egg-cell which precede 

 fertilization ; describe the process of fertilization and the 

 formation of the primary cell-layers, as exhibited in three 

 of the animal types known to you. What is the notochord, 

 and how is it developed in the frog ? 



2. (a) Explain what is meant by the term "central 

 nervous system." (3) Describe the tissue elements which 

 enter into its composition. (0) Explain, as far as you can, 

 the function of each structure described, (d) How is the 

 central nervous system developed in the frog, and (e) in the 

 rabbit? (/) What conclusions may be drawn from the 

 facts stated as to the origin of the central nervous system 

 in evolution? 



3. Describe the structure and cleavage of the ovum (a) of 

 the frog, (i) of the fowl, and (c) of the rabbit, (d) Explain 

 as far as possible the differences in the cleavage of these 

 three eggs, (e) Point out how the embryo is nourished in 

 each case, and (/) describe the constitution of the placenta 

 in the rabbit. 



