DEVELOPMENT IN PATIRIA MINIATA. 3 



stage the cells about the animal pole commence to elongate and, in 

 the gastrula, form a thickened area, more or less lens-shaped in 

 form, having approximately one half the diameter of the transverse 

 axis of the embryo. As indicated in the drawings (Figs, i, 2), 

 its center is exactly opposite to the blastopore and therefore is 

 strictly apical." Heath notes later that " all of the cells of the 

 apical plate are packed with granules, evidently yolk." 



That Heath was entirely incorrect in his interpretation of the 

 structure called by him the " apical plate " is easily shown. The 



3 



FIG. 3. A young Blpeiuiaria; a.p., "apical plate"; a.v., "anterior ves- 

 icle"; l.p., left posterior enterocrel. (After Heath.) 



fact that the cells of this structure are packed with yolk granules 

 should have given him a clue to the real condition ; for yolk gran- 

 ules belong to the vegetal pole, not to the animal or apical pole. 

 What Heath saw and accurately described were almost certainly 

 subnormal or inhibited larvae resulting from parthenogenetic eggs 

 or from eggs inhibited by poor environmental conditions. These 

 never develop normally, but always lag behind and exhibit various 

 anomalous structures. Figures 4-12, inclusive, are camera draw- 

 ings of a typical series of gastrulae derived from parthenogenetic 

 eggs of Patiria, reared by the writer in finger-bowls in the Pacific 

 Grove Laboratory. These larvae are all of the same age, about 

 two days old, and are comparable with Heath's first figure (Fig. 



