NO. 3 



INSECT HEAD SNODGRASS 



may arrive at a satisfactory conclusion as to the steps by which the 

 complex head of an insect has been evolved— and even then we must 

 allow much for errors of judgment. 



CEPHALIZATION 



It has been but little questioned that the numerous groups of meta- 

 zoic animals are derived from a creature resemliling the 1)lastula of 

 embryonic development (fig. i A). The embryonic blastula is exem- 

 plified, among living animals, in the early stage of the free-swimming 

 larval'planula of the Coelenterata (fig. 2 A). The planula develops 



Bid 



Fig. I. —Typical early stages in general embryonic development. 

 A hlastula stage diagrammatic, consisting of a blastoderm (SW) surround- 

 ing a blSocoele' cavity" (BIc). B, C, D,. stages in development of a chUon 

 Cfrotn Kowalevskv i88^) : B, d fferentiation of cells m blastula, L, gastruia 

 ^,on 7oS gast icoele cavit; (GO. lined with endoderm (tud), and opemng 

 through blaftfpore (Bp) ; D, later stage, showing ongm of mesoderm layers 

 (Msd) just within lips of blastopore. 



directly from the coelenterate egg, and has the form of a hollow mass 

 of cells the outer surface of which is covered with vibratde cilia. 

 The uniform motion of the cilia propels the animal through the water 

 in the direction of one axis of the body (fig. 3), and thereby one end 

 is distinguished as anterior and the opposite as posterior. The creature 

 thus becomes uniaxial and bipolar, though as yet there may be no 

 differentiation of body structure. The functional differences at the 

 two poles of the body, however, determine the course of the subse- 

 quent development of physical characters. Structural dififerentiation 

 of the end of the body that is forward in usual progression is called 

 cephalization, a term meaning the process of evolving a head. 



