STRUCTURE AND DEVELOPMENTAL RATE 231 



acy by continuous growth, it be injured or pinched away at an 

 early stage, the lateral buds very quickly grow out, showing their 

 liberation from some controlling influence possessed by the apical 

 bud. In other words, each growing hud {also true of the embry- 

 onic organs) exerts a depressing influence on the growth of all other 

 buds in the individual plant. As a shoot gradually ceases to grow 

 its depressing influence also gradually ceases. 



b. The initial linear growths, subsequent lateral buds, and the 

 interactions among the organs of the vertebrate embryo 



When the first trace of the embryonic body begins to express 

 itself in the blastodermic matrix it appears as a linear growth, the 

 head process extending forward from the blastopore or primitive 

 streak. This very soon becomes surrounded by, or associated 

 with the linear outline of the arising neural folds, the beginning 

 central nervous system. 



The neural folds indicating the early nervous system are origi- 

 nally of more or less straight outline and their first growth is largely 

 a growth in length. When in a given species the neural groove 

 has attained a certain length, it then begins a series of lateral 

 outgrowths, or branches. The first and largest of these are the 

 two optic outpushings and after them follow in a general way, a 

 series of bilateral outgrowths designated the three primary brain 

 ventricles. 



The initial linear origin and growth of the nervous system is 

 very probably due to an equal rate of cellular proliferation along 

 the entire extent, with perhaps a somewhat more rapid rate 

 at the tip. The lateral outgrowths arise on account of an exces- 

 sively high rate of proliferation occurring in a given region during 

 a certain time. For some unknown reason the rate of metabolism, 

 or actually the rate of oxidation becomes disproportionately high 

 in a particular group of cells, and these begin to multiply rapidly 

 as compared with the multiplication rate of neighboring regions, 

 and thus a folding or outgrowth occurs to produce, for example, 

 the optic vesicles. Since other portions of the brain seem nof to 

 be proliferating so rapidly at the same moment, it may be that 



