6 4 



JOHN W. MACARTHUR. 



of the gastrula wall and the other of the gut entoderm. These 

 two components may be of very different relative size (Fig. 2) 



J 



FIG. i. Blastulae and early gastrulae of the Atlantic sand-dollar, Echinarachniiis 

 parma. a-c, normal embryos; others, typical embryos as modified by lithium. In 

 the lithium series note: the overdevelopment and frequent basal extrusion of the 

 mesenchyme; the various degrees of enlargement of the sharply defined, thick- 

 walled endodermal portion; and the occasional occurrence (s, w, x) of partial cn- 

 dogastrulation. a and /8 arc double cxogastruhe, two joined terminally by their basal 

 ends. 



as might be expected from the mode of their formation. Some- 

 times the ectodermal portion exceeds the entodermal, often tln-v 

 are more or less equal, but in a great many the entoderm is 

 clearly much the larger. In the last mentioned cases the ccto- 

 dermal component becomes less and less, and approaches the 

 vanishing point a little knot of cells or nothing at all! while 

 the archenteron undergoes a compensatory and reciprocal hy- 



