242 THE POPULAR SCIENCE MONTHLY 



and posterior ends — are the earliest recognizable features of organiza- 

 tion of bilateral animals, and they appear while the germ is still uni- 

 cellular. The distinction between outer, intermediate and internal 

 organs next makes its appearance, each at first as a single tissue. The 

 outer tissue then separates into an epidermal and a nervous tissue, the 

 inner tissue into the intestinal and yolk-sac epithelium, the middle 

 tissue into muscle-forming tissue, connective tissue, skeleton-forming 

 tissue, blood-forming tissue, excretory tissue, peritoneal tissue, etc. 



For every structure, therefore, there is a period of emergence from 

 something more general. The earliest discernible germ of any part or 

 organ may be called its primordium. In this sense the ovum is the 

 primordium of the individual, the primitive outer tissue the primordium 

 of all structures of the skin and nervous system, the primitive inner 

 layer of the intestine and all structures connected with it, etc. Pri- 

 mordia are, therefore, of all grades, and each arises from a primordium 

 of a higher grade of generality. 



The emergence of a primordium involves a limitation in two 

 directions: (1) it is itself limited in a positive fashion by being re- 

 stricted to a definite line of differentiation more special than the 

 primordium from which it sprang, and (2) the latter is limited in a 

 negative way by losing the capacity for producing another primordium 

 of exactly the same sort. The advance of differentiation sets a limit, 

 in the manners indicated, to subsequent differentiation, a principle 

 that has been designated by Minot the law of genetic restriction. This 

 in a merely descriptive way is one of the general laws of individual 

 development, and in it is involved the explanation of many important 

 data in the fields of physiology and pathology. 



But, though primordia are thus restricted, they nevertheless have 

 the very important property of subdivision, in many cases at least, 

 each part retaining the qualities of the whole. Thus, for instance, in 

 some animals two or several complete embryos may arise from parts 

 of one ovum. Similarly, two or more limbs may be produced in some 

 forms by subdividing a limb bud. Thus frogs with six hind legs have 

 been produced by Gustav Tornier by the simple process of dividing 

 the primordia of the hind legs with a snip of the scissors, in which case 

 he found that on each side one part of the primordium produced a com- 

 plete pair of legs and the other the normal leg of that side. This 

 capacity for subdivision of primordia explains large classes of patho- 

 logical facts — at the same time it furnishes a problem to the student 

 of the physiology of development which has proved a serious stumbling 

 block. 



2. Principle of Organization. — I have already indicated the exist- 

 ence of direction and localization in the primordial germ of the 

 individual, the unsegmented ovum; the ovum, as we say, is polarized, 



