EXTERNAL INFLUENCES AS CAUSES OF VARIATION 279 



and galls, here was growth without systematic differentiation ; 

 cell division without the formation of an embryo. 



Encouraged by this degree of segmentation and by his experi- 

 ments upon irritability of muscle, Loeb tried a great variety of 

 solutions, in various degrees of concentration, in the hope of 

 carrying the segmentation far enough to produce real embryos 

 and live, free-moving larvae. 



He was greatly hampered by the fact that unfertilized eggs 

 do not form membranes as do fertilized, so that growth tended 

 to be formless, and even when assuming definite form in the 

 blastula 1 stage there were often formless masses of dividing 

 matter lying to one side. 



Briefly stated, the following facts developed during the prog- 

 ress of this systematic experiment : 



1. In a solution of sodium chlorid the eggs were unable to 

 reach even the blastula stage. 



2. With the addition of MgCl 2 , however, blastulae were formed, 

 but they did not move. When afterward placed in normal sea 

 water movement soon appeared. 



3. With three chlorids (Na, K, and Ca) "the eggs not only 

 reached the blastula stage and swam around in the most lively 

 way, but they reached the gastrula and even the pluteus stage, 

 with the exception, however, that practically no skeleton was 

 formed." 2 Such larvae lived about ten days. 



4. The addition of a trace of Na 2 CO 3 resulted in the formation 

 of a skeleton, but it was not quite normal. It was made normal 

 by adding a trace of MgCl 2 . 



1 Three early stages are characteristic of the early development of all embryos : 

 (i) the morula, or "mulberry " stage, in which cell division gives rise to a globular 

 mass of rounded cells, each more or less distinct, like the grapes on a bunch or 

 the seeds of a mulberry ; (2) the blastula stage, in which the outer cells become 

 condensed, showing a distinct outer layer, the blastoderm ; and (3) the gastrula 

 stage, in which one side becomes pushed in (invaginated), as one would push in a 

 hollow rubber ball with his thumb, forming a kind of mouth and stomach. A few 

 forms never get beyond this stage, but most pass quickly through it, differentia- 

 tion proceeding rapidly. In higher animals the outer layer (ectoderm) gives nse 

 to the skin and its appendages, the inner (endoderm) to the internal organs. 

 Among sea urchins, which were here under experiment, the next stage is known 

 as the pluteits, the stage of free-moving larvae. It was this stage the experi- 

 menter desired to produce. 



2 Loeb, Studies in General Physiology, Part II, pp. 585~5 86 - 



