22 



HISTOLOGY 



from the methods used to meet similar conditions, and not from phylo- 

 genetic relationships, other than the common possession of a protoplasm 



which is subject to the same laws. 



The early bilateral symmetry of the 

 young eggs of many animals is a pre- 

 formation of the form of the body best 

 adapted to the needs of motion (Fig. 

 19). Where there is no ancestral his- 

 tory of motion, or where this history is 

 very far in the background, the animal 

 develops with an upper and a lower 

 pole and a number of radially symmet- 

 rical sides. The number of these sides 

 varies from four sides in some me- 

 dusse to the greater numbers seen in the 

 echinoderms or the theoretically infinite 

 number in the sessile sponges. 

 When the animal is one that is to be moving in any part of its life 

 history, the development tends toward the bilateral symmetry found 

 in all animals that move forward. The bilateral symmetry may be 

 superimposed upon the radial as is seen in the adults of some echino- 

 derms, where radial symmetry extends into many planes, while that same 

 radial structure is preceded by a bilateral form in the early embryo. 



In the smaller and less highly organized forms the tendency is to 

 form organs as soon as possible, that the creature may begin an inde- 

 pendent life, securing its food and escaping its enemies by various motor 

 and protective devices. These organs are often temporary, and replaced 

 by other forms in the adult. The further details of the development of 

 tissues will be considered, where they are necessary to a real under- 

 standing of histological structure, in connection with the descriptions of 

 the various organs. 



LITERATURE 



FIG. 19. Segmenting ovum of Loligo 

 Pealii to show early traces of bilateral 

 symmetry. X 45. (After WATASE.) 



Read the parts of Wilson, "The Cell," Schneider, " Lehrbuch der Histologie," and 

 other general works that cover this subject. 



