228 



INTERNAL FACTORS 



IV. i 



organ -form ing substances exist, though experimental proof is at 

 present lacking. 



In the immature egg of the aberrant parasitic Polychaet 

 Myzostoma there is, according to Driesch, at the vegetative pole 

 a mass of greenish cytoplasm, the surface of which is convex 

 towards the reddish brown mass of the rest of the egg. When 

 the polar bodies are formed the red substance retreats to the 

 animal pole, leaving a clear equatorial zone. 



Before segmentation a polar lobe is formed as in Ilyanassa 



FIG. 138. Development of the egg of Myzostoma. The green sub- 

 stance on the vegetative side is shaded finely ; this passes into the polar 

 lobe (e}, and into the D quadrant (i). The red substance of the animal 

 hemisphere is coarsely dotted. (After Driesch.) 



FIG. 139. Development of the egg of Myzostoma. Formation of the 

 cells 1 d (d 1 in c) and 2 d or X (d 2 in e). The latter cell takes a portion 

 of the substance of the polar lobe. (After Driesch.) 



and Dentalium containing the green and part of the clear 

 substance : it passes first to CD and then to D. When the latter 

 divides it is again protruded and withdrawn. The cell 1 d, like 

 1 a, \b, lc, contains the reddish protoplasm. The second micro- 

 mere in the D quadrant 2d takes part of the green substance, 

 and Driesch suggests that the rest of the latter would eventually 

 pass into the second somatoblast (4d = M) (Figs. 138, 139). 



