568 



COLLEGE ZOOLOGY 



embryo, Fig. 403, left side) . Such an opera- 

 tion is called embryonic transplantation. 

 Would the optic cup continue to develop 

 in its new location? If so, would it stimulate 

 the overlying ectoderm to form a lens? The 

 answer to both questions is yes. Thus it is 

 clearly evident that the optic cup does in- 

 deed exert some influence on the overlying 

 ectoderm which causes the latter to differen- 

 tiate into a lens. The process of stimulation 

 is called embryonic induction. This ap- 

 proach to the study of development which 

 revealed the role of embr\'onic induction in 

 the process of cellular differentiation of a 

 lens is called experimental embryology. By 

 systematic investigation of all other regions 

 of the developing embryo where similar 

 causal relations between adjacent groups of 

 cells were suspected, experimental embryolo- 

 gists have demonstrated convincingly that 

 interaction between adjacent groups of cells 

 does play an exceedingly important role in 

 embryonic differentiation in many species. 

 Having demonstrated the existence of em- 

 bryonic inductions, the next step was to 

 study the mechanisms of induction. Does 

 the optic cup produce a specific chemical 

 substance which diffuses to the nearby lens- 

 forming ectoderm and stimulates the latter 

 to differentiate into a lens? Or does the 

 optic cup stimulate the adjacent ectoderm 

 physically in some way? Similarly, experi- 

 mental embr}'ologists ask questions concern- 

 ing changes in the reacting cells during and 

 following embryonic induction. Increasingly, 

 today's approach to development is to iden- 

 tify and study the chemical and physical 

 bases of embr}'onic differentiation. 



Organizer theory 



It has been found that the very first ap- 

 pearance of a definite head-tail axis, and of a 

 nervous system, in the early embryo, is due 

 to interactions between mesoderm cells and 

 the overlying ectoderm. This was first shown 

 by Spemann, who was awarded the Nobel 

 prize for a scientific achievement of tre- 

 mendous importance. Spemann demon- 



strated this by transplanting the dorsal lip 

 of the blastopore (the opening into the 

 archenteron) of one salamander gastrula to 

 a site in another gastrula where a nervous 

 system would not normally form. It was 

 found that the transplanted dorsal meso- 

 derm (chordamesoderm) induced the ecto- 

 derm with which it was in contact to become 

 a neural tube. The transplanted mesoderm, 

 itself, self-differentiated into notochord, 

 somites, and other structures, so that a small 

 embryo was formed from the combination 

 of host and donor tissues. No other region 

 of the early gastrula was found to have this 

 inductive ability, so the chordamesoderm of 

 the salamander gastrula has been called the 

 organizer. Much important work still needs 

 to be done in this fascinating field of ex- 

 perimental embryology. 



METAMORPHOSIS 



Metamorphosis occurs commonly during 

 the development of invertebrates and occa- 

 sionally in vertebrates. By metamorphosis is 

 meant a sudden, marked change in the 

 course of development. Such changes occur 

 even among the protozoans, where, for ex- 

 ample, the ciliated embr^'os of Suctoria, 

 after swimming around for a while, settle 

 down, become attached, and transform into 

 tentacle-bearing adults. Similarly, the free- 

 swimming, flagellated embryos of sponges 

 metamorphose into permanently attached 

 animals with an osculum at the outer end 

 and spicules in the body wall (Fig. 49). 

 No free-swimming larva appears in the life 

 cycle of the hydra, but in many other 

 coelenterates, such as Obelia (Fig. 57) and 

 Aurellia (Fig. 60), a ciliated planula be- 

 comes attached and metamorphoses into a 

 hydroid. Many marine annelids and mol- 

 lusks pass through a trochophore stage 

 from which the adult develops. In many spe- 

 cies of starfishes, the egg develops into a 

 larva called a bipinnaria (Fig. 429), which 

 is bilaterally symmetrical; the free-swimming 



