1 88 Studies in Animal Behavior 



another. In the eggs of many insects where they are 

 differentiated very early in development they have 

 been observed actually to pass outside the embryo 

 for a time only to wander back again. Hegner has 

 performed the experiment of removing these cells 

 at the time they passed out of the egg and found 

 that no sex cells developed in the resulting ani- 

 mal. In the embryos of several of the -lower ver- 

 tebrates Dr. B. M. Allen has found that the sex cells 

 arise in the entoderm. Then they wander out along 

 the rudiment of the mesentery and take up their 

 final position at a considerable distance from their 

 point of origin. As to what causes the movements 

 of these cells and guides them in their course we 

 can only offer but very tentative conjectures. 



One of the most remarkable features of develop- 

 ment is the way in which nerve fibers grow out 

 from the central nervous system and push their way 

 between various masses of cells to become con- 

 nected with the proper end organs. The organism 

 is permeated with a network of these fibers, spun 

 out as fine as gossamer threads and connected with 

 one another and with various parts of the organism 

 in a very intricate way. In order to build up this 

 elaborate and delicate system the nerve fibers as 

 they grow out from the developing brain and spinal 

 cord must take the right paths and connect par- 

 ticular groups of cells in the central nervous sys- 

 tem with corresponding parts of the body. Should 

 nerve fibers not find their proper termination all 



