I Nature's job in embryogenesis is to create a 

 young, functioning, multicellular organ- 

 ism. Her raw materials are ( 1 ) an egg, a 

 single cell, large with respect to sperm 

 but tiny with respect to the finished 

 product that is to come, and ( 2 ) a sperm 

 containing little more than a nucleus 

 which, when joined with the egg nucleus, 

 will provide the instructions needed to 

 produce the embryo. She solves the prob- 

 lems of embryogenesis by the following 

 strategems : 



1. Cleavage: the fertilized egg is 

 first subdivided into a large number of 

 smaller cells, because little self-contained 

 packets are easier to control and alter 

 than a single, undivided mass of proto- 

 plasm. 



2. Gastrulation: the cleaved cells 

 are moved about so as to create three 

 basic tissue layers. These become further 

 subdivided and give rise to the tissues 



of t h 6 and organs of the functional adult. 



3. Organ formation: each subdivi- 

 sion of the three original tissue layers be- 



Embrvo comes a semi-autonomous system within 



which cells appear by division, are 

 grouped into tissues, and finally become 

 functional entities ( kidney, spleen, brain, 

 etc.). Thus, many changes occur simul- 

 taneously within the subsystems of the 

 developing embryo, and they must be 

 linked in such a way as to operate har- 

 moniously, in the right place at the right 

 time. This is accomplished in part by 

 having chemical signals pass between de- 

 veloping subsystems so that one of them 

 47 



The 

 Development 



Vertebrate 



