84 JOHN W. MACARTHUR. 



are few and large while the entodermal layer is rich in smaller 

 nuclei. The mesenchyme cells proliferate in enormous excess 

 and fill and overdistend the small blastocoel. Thus considera- 

 tions of space and inner pressure alone render almost mechan- 

 ically impossible the infolding of the entoderm which is, further, 

 much longer, larger and thicker than usual, and is contiguous 

 with the ectoderm by a vastly enlarged blastopore ring. 



Certain other relevant facts should be noted: that the lumen 

 of the exogastrular gut is really a part of the blastocoel or coelome 

 and is not the true gut cavity; that the constrictions of the dif- 

 ferentiating gut are also therefore reversed in direction; that the 

 stomadeum also often everts in starfish; that exo- and endo- 

 gastrulation intergrade (there may be first one and then the 

 other, or both at once). It is possible that the agent in obliterat- 

 ing and reversing the primary physiological gradient of the or- 

 ganism as a whole at the same time obliterates and reverses the 

 functional polarity of the separate cells of the epithelial layers, so 

 that insofar as polarity of cells aids in normal endogastrulation it 

 might act in the opposite way in the lithium larvae (by failure 

 of the cells to widen and vacuolate basally, change their nuclear 

 position, etc. cf. Gudernatsch, '13). 



Tests of this interpretation of the lit hi um action may be made 

 by an appropriate modification of the technique on other gradi- 

 ents (embryological, regenerating, motor). This agent might 

 tend to reverse the polarity of regenerating pieces (e.g., hydroid 

 internodes, as an electric current does, Lund, '21), or of motor 

 gradients (cilia, or muscular peristalsis), etc. 



SUMMARY AND CONCLUSIONS. 



1. Typical lithium larvae may be produced from a high per- 

 centage of eggs of the sand-dollar, Echinarachniiis parma, and 

 of two sea-urchins, Arbacia punctulata, and Strongylocentrotus 

 francisciana, by addition of LiCl to the sea water. These types 

 approximate closely those obtained by Herbst in other urchins. 



2. Morphological features common to and characteristic of 

 these larvae are (p. 78): (a) exogastrulation in many cases, and 

 (b) always increase of mesenchyme and of entoderm and a com- 

 pensating decrease of ectoderm. Kurt her tin- most apical p.iris 

 of the ectoderm and its differentiations are most decreased, and 



