152 



EMBRYOLOGY 



ORIENTCO BY STRETCHING 



Fig. 89. Orientation of long, 

 narrow particles, and the results of 

 such orientation on the growth of 

 nerve fibers. A: A culture of neuro- 

 blasts in an unoriented medium. 

 Nerve fibers grow out in all direc- 

 tions. B: The medium is oriented by 

 stretching, and the nerve fibers then 

 grow parallel to each other. C: Two 

 cultures orient the intervening me- 

 dium and nerve fibers grow in paral- 

 lel fashion. 



ORIENTED BY GROWTH OF TISSUES 



follows: the particles are oriented at random to begin with, but with stretch- 

 ing each one begins to turn and they become oriented in parallel fashion, as 

 in Figure 89, C. 



An understanding of these theories is of the utmost importance, since 

 the directed movements of cells during embryonic development is very wide- 

 spread. Here are but a few examples: the growth of blood vessels to an organ 

 and the branching and growth of vessels within an organ; the growth of 

 the mesonephric duct to join the cloaca; the union of the epidermal half of 

 the pituitary gland with the nervous half; the migration of some cells of the 

 neural crests to form the sympathetic ganglia while other cells migrate to the 

 feather germ and deposit pigment; the union of the ectodermal half of the 

 adrenal gland with the mesodermal half; the growth of the ureter from the 

 mesonephric duct. There is indeed an impressive array of processes in which 

 cell migration occurs. This migration must be directed by one or more of 

 the three factors which we have considered. 



At present there is no good way of determining which of these three 

 theories is correct. There are arguments for and against each of them. This 

 statement may seem anticlimactical, but the student must realize the diffi- 

 culties of research in embryology and he must be prepared for such gaps in 

 our theory. 



