198 



INTERNAL FACTORS 



IV. i 



ectoderm, the mesenchyme, the archenteron. This assertion of 

 Driesch's is corroborated by Morgan for the whole larvae, but not 



for the organs ; according to him 

 the number of cells invaginated to 

 form the archenteron in a partial 

 tends to approach that found in 

 a whole larva, about fifty. 



Driesch has also determined 

 the relation to their germinal 

 value of the dimensions of partial 

 blastulae, gastrulae, and Plutei 

 and their organs (Figs. 108- 

 112). As the table (Table XX) 

 shows, the surfaces are very 

 nearly as the germinal values, 

 the ratios of the radii of the partial and total larvae con- 

 sequently greater, of the volumes less than these values. 



FIG. 108.- Outlines of }, \, J, 

 and -J blastulae of Sphaerechinus. 



(After Driesch, 1900.) 



FIG. 109. Early gastrulae of Echinus: a. J, I. |, c. J-, d. 

 (After Driesch, 1900.) 



