RECONSTITUTIONAL PATTERNS IN EXPERIMENT 427 



centrifuged before or after fertilization, the centrifugal region tends to 

 become ventral. ■'" According to Lindahl, however, in centripetal frag- 

 ments of these eggs the ventral side is centripetal, and in another species 

 he finds the ventral side centripetal. Also, in eggs stretched by being 

 drawn into a capillary tube the end in advance becomes ventral, because 

 greater stretching of this end renders plasma colloids less stable. Deep 

 staining of this end with Nile blue sulphate makes it dorsal. Lindahl sug- 

 gests that the ventral region is an inductor and has a higher metaboHsm 

 than the dorsal region and that the region where protoplasmic inclusions 

 are aggregated becomes ventral because the inclusions determine higher 

 metabolism in it. Differential dye reduction and susceptibility agree in 

 indicating or suggesting higher metabolism in the ventral region in normal 

 sea-urchin embryos.^^ But why the ventral region should be centrifugal 

 in the whole egg and centripetal in a centripetal piece is not entirely clear. 



Unfertihzed Dendraster eggs centrifuged 6-8 minutes at 45,000 times 

 gravity develop into plutei with an undivided centripetal lobe, situated 

 ventrally at the angle between ventral and anal surfaces in 93.6 per cent 

 of the larvae. Of these, 56.9 per cent have the lobe lateral on the ventral 

 surface, in 14.9 per cent it is nearly median, in 22 per cent intermediate 

 (Pease, 1939). Ventrodorsality is believed to be present in the egg and 

 to be shifted by centrifuging. A "ventral-determinant" gradient is postu- 

 lated with highest concentration or activity ventral, probably cortical, 

 perhaps an enzyme, requiring a substrate probably dififuse in the ento- 

 plasm but concentrated centripetally by centrifuging and so partially ro- 

 tating the ventrodorsal axis. In centrifuged eggs of the gephyrean Urechis 

 the centripetal region also tends to be ventral and without relation to 

 point of sperm entrance or first cleavage (Pease, 1938). The "determi- 

 nate" cleavage pattern of ultra-centrifuged eggs of the pelecypod Cumin- 

 gia and the polychete Chaetopterus is related to the stratification, though 

 with wide variation in Cumingia; and polarity and bilaterality are ap- 

 parently determined in relation to the cleavage pattern. ^^ 



Eggs of the sea urchin Arhacia suspended in sugar solution of proper 

 density can be separated by strong centrifuging into two parts, the cen- 

 tripetal part colorless, the centrifugal granular and pigmented; and each 

 of these parts can be again separated by further centrifuging. These egg 



32 Runnstrom, 1925c, 1926a; Lindahl, 1936. 

 3-5 See pp. 134-38 and chap. vi. 



i"^ Pease, 1940, "The influence of centrifugal force on the bilateral determination and the 

 polar axis of Cumingia and Chaetopterus eggs," Jour. Exp. Zool., 84. 



