84 



THE CELL AND PROTOPLASM 



esses of differentiation. The methods used 

 are similar to those first successfully em- 

 ployed with amphibian eggs, i.e., transplan- 

 tation and vital staining, besides methods 

 of separating individual blastomeres and 

 parts of older embryos with due regard to 

 the exact origin of each cell or cell group 

 used in the experiments. 



The sea-urchin egg shows graded differ- 

 ences along its main axis, about which the 

 cytoplasmic substances appear superficially 

 to be radially symmetrical (Boveri 1901a). 

 The polar axis is not shifted by centrifug- 

 ing but its degree of fixity is a matter of 

 dispute. Tennent, Taylor and Whitaker 

 (1929) claim that in Lyt echinus it may be 

 shifted by cutting the egg, while Horstadius 

 (1937) finds that in the egg of Arhacia it 

 is fixed. Development of egg fragments 

 shows, too, that even in the unfertilized egg 

 there is a considerable degree of cyto- 

 plasmic localization. While the eggs of but 

 few echinoderms have actually been shown 

 to be bilaterally symmetrical, with dorso- 

 ventral polarization, this is probably true 

 for the group generally, though the plane 

 of symmetry may be shifted and the polar- 

 ity of the dorso-ventral axis fixed by such 

 external factors as stretching (Boveri 1901; 

 Lindahl 1932), centrifugation (Runnstrom 

 1926; Lindahl 1932a; Pease 1939) or chem- 

 icals (Lindahl 1932). However, the details 

 are not all in agreement. When the egg of 

 Dendraster, the Pacific Coast sand dollar, 

 is centrifuged, the centripetal pole is found 

 to lie on the ventral side though frequently 

 not in the median plane. Pease concludes 

 from his experiments on this form that two 

 factors are concerned in fixing the dorso- 

 ventral axis, one, located in the cortical 

 layer, which is not movable, and the other, 

 located in the endoplasm, which may be 

 shifted by centrifuging. The direction of 

 the axis, and with it the median plane, is 

 the resultant of the interaction of the two 

 factors, the ventral side being in the region 

 of maximum reactivity. When eggs are 

 cut in the frontal plane during cleavage 

 stages, the dorso-ventral polarity is re- 

 versed in the dorsal half (Horstadius and 

 Wolsky, 1936). The bilateral symmetry of 



the embryo usually first becomes visible 

 when the primary mesenchyme cells group 

 themselves on the two sides, prior to laying 

 down the larval skeleton, in response to 

 stimuli emanating from the ectoderm 

 (Driesch, 1896). 



By means of isolation and grafting ex- 

 periments Horstadius (1935) has made an 

 exhaustive analysis of the action of the 

 material in the successive layers along the 

 primary axis of the egg. Of great interest 

 are the cases in which the segmenting egg 

 of from 16-24 cells is divided "horizon- 

 tally" by tiers, which are allowed to de- 

 velop by themselves or in various com- 

 binations. The results show that the 

 combination must contain material from 

 both the animal and vegetative regions of 

 the egg in proportions that do not deviate 

 too far from the normal. An animal half 

 of the egg or the uppermost tier of cells 

 in the animal hemisphere gives rise to a 

 blastula in which the apical tuft of stiff 

 cilia is much enlarged. If micromere cells 

 from the opposite pole are added, the tuft 

 is reduced to proper proportions and a nor- 

 mal small pluteus may develop. In fact, 

 two normal plutei may be obtained from 

 a single egg after horizontal division, pro- 

 vided the middle part is left intact and the 

 micromeres are joined to the "most ani- 

 mal" cells. This involves a considerable 

 amount of regulation, in which particular 

 cells give rise to structures quite different 

 from those they would normally form. 

 Animal (ectodermal) cells may be endo- 

 dermized by the presence of micromeres, 

 and a certain amount of vegetative material 

 is necessary to inhibit the spread of the 

 apical tuft. On the other hand some ani- 

 mal material is necessary for gastrulation 

 to take place. 



In such processes of differentiation there 

 must be a lively reaction between the 

 changing cell and its internal environment 

 of other cells. This must either take place 

 through direct contact, as when the micro- 

 meres induce presumptive ectoderm to in- 

 vaginate (gastrulation), or be mediated by 

 substances diffusing through the blastocoele 

 from cells at a greater distance, e.g., in the 



