482 



ZOOLOGY 



non-sexual reproduction), no matter how high in the scale 

 of life, starts its life as a single cell, similar in many respects 

 to the permanent single-celled protozoans, except that it has 

 the power of developing rapidly into its own peculiar species. 

 (See Fig. 250, 7.) 



FIG. 250. 

 Mammalia, 



<5. XfotochorA 



3. Sey mentation* 



. Gtzstrulet 



l.O/ie-celted, 



FIG. 250. A diagram to suggest the way in which individuals of the different phyla have 

 parallel development for a while, and later diverge in their own particular way. For example, an 

 Amphibian begins life as a single cell, similar to the simple protozoa (i); later it forms a mass of 

 cells similar to some of the colonial protozoa (2) ; in forming a gastrula (3) it is going through a 

 stage similar to the Hydra, which is a kind of permanent gastrula. Amphibians run parallel to the 

 fishes in development, in the formation of a body cavity (4), in forming segments (5), in develop- 

 ment of notochord (6), in the presence of gills in early life (7); but part company with the fishes in 

 the fact that they lose the gills and develop lungs (8). 



Questions on the figure. Trace out other instances of parallelism until the 

 purpose and meaning of the diagram become clear. Then find other facts with 

 respect to development of animals and show how the diagram cannot do justice 

 to the whole truth. Compare diagram with text, sections 486 to 487. 



2. All higher forms agree likewise in the next step of their 

 development, which consists in the division of this single cell 



