282 ELEMENTS OF BIOLOGY 



ture to the endodermal pouches in the fish embryo. In the fish they 

 develop into gills; in Man they develop into a number of struc- 

 tures. Human embryos do not develop gills. 



Form and Symmetry in the Developing AnimaL It has 

 been possible here to consider only the high lights of the essentials 

 of development of the body layers and organ systems of the animal. 

 The account would be still more incomplete if we did not con- 

 sider the question of the manner and mechanisms bv which the 

 new individual is organized into an animal having a definite an- 

 tero-posterior axis, an anterior and a posterior end. with organs and 

 limbs appearing at definite places along this axis and with right ■ 

 and left halves essentiallv mirror images of each other. This con- 



, CI 



trolling and determining nature of the developmental process al- fl 

 ways interests the professional biologist; it may be that the general 

 reader will be similarly intrigued once the nature of the problem is 

 made clear. 



To begin with, protoplasm is not a chaotic association of water, 

 proteins, fats, carbohydrates, salts, et cetera, but is, as we have seen 

 (Chapter I) a highly and intricately organized polyphasic colloidal 

 system. Moreover, the cell is a similarly highly organized unit com- 

 posed of this complex colloid, each type of cell being somewhat 

 differently organized although all cells have features in common. 

 The adult animal is likewise not a fortuitous association of cells, 

 tissues, and organ systems no more than a house is a mere mass of 

 bricks, mortar, wood, and iron. One may therefore speak of the 

 type of organization of the animal as its structural pattern. Atten- 

 tion will be confined here to the tvpe of pattern that is exhibited 

 by the higher animals, characterized by an anterior-posterior axis 

 and bilatetral symmetry. 



When the fertilized t^^ starts to develop the first signs of division 

 are noted in the upper, more active, animal pole. The cleavage 

 planes are initiated there and hence this region acts as a leading 

 element during the early stages of development. Similarly, through- 



