488 



ZOOLOGY 



vestigial organs are also evidence of evolution of animals into 

 adjustment with the surroundings. 



486. Evidences from Embryology. — The study of em- 

 bryology — the course of life in the individual — has probably 

 furnished us the most suggestive evidence of the evolution of 



Pig. 250. 



:Birda 



Wmphibia. 



J?hs/tes 



Glands "^ 



8£ossaf€ias 



rGiXSlUs 



6.7}btochor3. 



dSeffrmniaiion' 



•3. Gctstrula, 



^. JiZorulA 

 J.One-cdi^d. Ovum. 



K 



^ tOormSi-anniUaia. 

 SchuwderfnS 

 •vtG/vlnx, etc. 



Colonial J^re^exjo. 



j3u^ie-ce?ied J^atoi 



Fig. 350. 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 and 487. 



animals. The main facts, and the use that has been made of 

 them in reaching our conclusions, may be expressed in a few brief 



