512 PATTERNS AND PROBLEMS OF DEVELOPMENT 



the micromeres apparently bring about increase in size of entoderm, 

 whether by increased activation of the basal region or otherwise. 



Other experiments show entodermization of presumptive ectoderm in 

 somewhat different ways. A small group of cells derived from arh, stained 

 for identification and implanted in the basal region, may become part 

 of the entoderm. Combination of the eight apical cells of the sixteen-cell 

 stage (stained) by their basal surfaces with the median surface of a merid- 

 ional half results in larvae in which stained cells of presumptive ectoderm 

 form almost half of the entoderm (Horstadius, 1928a, b; 1935). Here 

 either vegetal substance presumably diffuses from the basal region of the 

 meridional half into presumptive ectoderm of the apical half and ento- 

 dermizes it or the activation in the basal region of the one induces the 

 secondary gradient in presumptive ectoderm of the other, either dynami- 

 cally or by diffusion. 



The preceding experiments have been concerned with apicobasal pat- 

 tern only. Certain others, bearing on the problem of ventrodorsality, are 

 also of interest. According to earlier authors, 1/2 and 1/4 blastomeres 

 reconstitute whole larvae; but certain defects, particularly in skeletons 

 of larvae from isolated 1/2 blastomeres, were described by Plough (1927, 

 1929). Halves and quarters of the earlier cleavage stages of Paracentrotus, 

 isolated in planes of first and second cleavages, may give normal or prac- 

 tically normal plutei — in fact, four almost completely normal plutei may 

 develop from a single egg (Horstadius und Wolsky, 1936). Orientation by 

 staining of the surfaces of separation of these halves and quarters leads 

 the authors to conclude that first and second cleavages may have any 

 relation to the median plane, that ventral halves and quarters develop 

 more rapidly than dorsal, and that the original ventrodorsality persists 

 in ventral but is inverse in dorsal halves and quarters, the original dorsal 

 side becoming ventral, the original ventral, dorsal. The more rapid de- 

 velopment of ventral isolates is to be expected since the high region of the 

 ventrodorsal gradient is ventral (pp. 134-40). Lindahl (1932) and Hor- 

 stadius have also suggested a higher metabolism ventrally than dorsally. 

 Inversion of ventrodorsality in the dorsal half or quarter apparently rep- 

 resents activation of the low region of the ventrodorsal gradient in conse- 

 quence of isolation; possibly inhibition by staining of the surface of sepa- 

 ration may also be concerned in bringing about the inversion. Right and 

 left halves, stained on the surfaces of separation, are usually defective on 

 the stained side. Isolated meridional eighths of thirty-two-cell stages 

 without micromeres reconstitute larvae approaching pluteus form more 



