332 



Embryogenesis: Progressive Differentiation 



any point and since the fust cleavage plane 

 passes througJi tnis point, the structure ol 

 tlie ovocyte ox Nereis at the time oi insemina- 

 tion must be tlie same in all meridians. 

 Morgan and lyler {'60) have investigated 

 this relationship in more detail in annelids 

 and mollusks. Ditferences of opinion exist 

 as to the relation between the sperm en- 

 trance point and the median plane in the 

 sea urchin egg. Horstadius C'39) stained the 

 side opposite the point of sperm entry and 

 found this point in all positions in relation 

 to the first cleavage furrow. Since the lirst 

 cleavage furrow in sea urchin eggs has no 

 rigid relation to the futme median plane, 

 it remains uncertain how the sperm entrance 

 point is related to the median plane. Lillie 

 ('09) examined development of fragments 

 of unfertilized Chaetopterus (aruielid) eggs 

 separated by centrifugation. He believed that 

 his study of cleavage pattern in these frag- 

 ments indicated that bilaterality developed 

 subsequent to their fertilization. He con- 

 cluded (p. 65), "... if bilaterality can de- 

 velop in the parts there is no reason for 

 assuming its prior existence in the whole; 

 and its origin must be regarded as a purely 

 epigenetic process." Similarly Pease ('38), 

 upon discovery that he could control the 

 position of the ventral surface in eggs of 

 the armelid Urechis by centrifugation, con- 

 cluded (p. 422), "It seems best to regard this 

 bilateral determination as induced in the 

 egg and not as a rotation or shift of a 

 bilateral axis or predisposition already pres- 

 ent in the egg, although such may be present 

 in a weakly defined form." 



The most detailed studies of determina- 

 tion of dorsoventrality are those on sea 

 urchin and sand dollar eggs. If sea urchin 

 eggs are stretched perpendicular to the ani- 

 mal-vegetal axis by forcing eggs through 

 a fine capillary tube (Lindahl, '32a) or by 

 constricting them meridionally by a single 

 silk fiber (Horstadius, '38), the axis of 

 stretching becomes the dorsoventral axis. In 

 the experiments with the capillary tubes 

 Lindahl found that the end of the stretched 

 egg which usually develops into the ventral 

 surface becomes the dorsal surface instead 

 if it is overstained with vital stain. Simi- 

 larly in centrifugation experiments the 

 dorsoventral axis coincides roughly with the 

 axis of centrifugation (Runnstrom, '26; Lin- 

 dahl, '32b; Pease, '39). Pease concluded from 

 his experiments on sand dollar eggs that 

 two factors are involved in ventral deter- 

 mination: (1) a ventral determinant present 

 in gradient form with its greatest intensity 



Oil ine prospective ventral side; this he con- 

 siuerea lo ue locatea in uie egg cortex axiu 

 liOL sniiiea uy ceiuruugauoii, (^^) a suo- 

 buate locateu in tne enLopiasiii, uiiuormj.y 

 uisuiouiea m a normal egg, duc concemrated 

 a I tne centripetal poie in cenuiiugea eggs. 

 jL^e buggesieu tnai ihe ventral aeierminani 

 iiiay ue an enzyme adsorbed in tne corucax 

 xuyx^ia. xxe proceeaed to test tnis hypomesis 

 (^ ti, 42a, d; by exposing eggs m tne o-ceii 

 stage to gradients ot Known enzyme in- 

 niDitors. Vvith certain specihc enzyme inhiD- 

 iiors me ventral surtace tormed only on tne 

 side away irom the source of the inhibitors, 

 suggesting that the enzyme systems con- 

 cerned specihcally with dorsoventral deter- 

 mination will cause a ventral side to form 

 wherever their activity is greatest. It has 

 been known for a long time that many 

 chemical treatments of echinoderm eggs com- 

 pletely suppress bilaterality and produce 

 radially symmetrical forms; more recent ex- 

 periments by Rulon ('49, '51, listed in Rulon, 

 '52) with specific enzyme inhibitors, fol- 

 lowed by return of the treated eggs to sea 

 water, seemed to cause a spread of ventral 

 determination over a greater portion of the 

 egg than usual, i.e., they caused a "ventrali- 

 zation" of the egg. 



If sea urchin eggs are separated meridi- 

 onally in advanced blastula stages, and the 

 separation surfaces are vitally stained, some 

 pairs of larvae are obtained in which one 

 twin is stained dorsally, the other ventrally 

 (Horstadius and Wolsky, '36). This would 

 be the expected result if dorsoventrality 

 were already determined at the time of the 

 operation and if the cut happened by chance 

 to pass through the frontal plane. By con- 

 trast, if the first two blastomeres are sepa- 

 rated, followed by staining of the surface of 

 separation, pairs of larvae are found in 

 which both are stained dorsally. One larva 

 in such a pair tends to develop more slowly 

 and to form less perfect ventral structures 

 than the other. Presumably the latter repre- 

 sents the prospective ventral half of the egg, 

 the former the prospective dorsal half, and 

 the first cleavage plane happened to pass 

 through the prospective frontal plane. If so, 

 the original dorsoventral axis has been re- 

 tained in the prospective ventral half, re- 

 versed in the prospective dorsal half. Since 

 such a reversal of dorsoventrality is possible 

 in early stages, and not in later ones, estab- 

 lishment of a rigid dorsoventral organization 

 must be a progressive process. Similar re- 

 suits have been obtained by Gustafson and 

 Savhagen ('50) using anionic detergents (see 



