374 HAROLD H. PLOUGH. 



the first two cleavage planes cut this differentiated zone at right 

 angles, and that the first cleavage plane marked the median plane 

 of the larva. In support of this idea he pointed to the fact 

 brought out by Driesch himself that the isolated animal half of 

 the transversely divided eight cell stage seldom gastrulated, 

 while the isolated basal half (vegetative portion) more often 

 developed normally. Driesch did not accept this interpretation, 

 and in 1906 described the development of an egg incompletely 

 separated in the two cell stage but differentiating as one individ- 

 ual. Here he found that the skeletal rudiments appeared in the 

 descendants of one only of the original cells, and he concluded 

 therefore that the first cleavage plane cut at right angles to thi 

 median plane of the larva, and that, contrary to Boveri, the egg 

 was first divided into animal and vegetative portions. 



Here the matter has stood up to the present. The majority 

 opinion has accepted Boveri's evidence, as shown by Conklin's 

 summing up of the situation (1924, page 586): "It is plain that 

 there is a differential distribution of egg substances to the cleavage 

 cells from the animal to the vegetative poles, though no differen- 

 tiation of cells in any cross axis can be detected until much later." 

 E. B. Wilson (1925, page 1067) said: "These facts (e.g., of Boveri) 

 demonstrate that the sea-urchin egg is no more isotropic than 

 that of the mollusk or annelid." 



The experiments of Von Ubisch mentioned above prove that 

 both Driesch and Boveri were right with respect to the relation of 

 first cleavage plane and the median plane of the embryo, yet in 

 spite of this are in no sense incompatible with Boveri's idea of a 

 differential organization of the egg at right angles to the initial 

 egg axis. This investigator has succeeded, where Boveri failed, 

 in staining one or more of the blastomeres in dividing eggs of 

 Echinus and Echinocyanus and then noting the portions of the 

 larva? which are marked with the dye. By this method he has 

 demonstrated quite conclusively that there is no constant rela- 

 tion between the first cleavage plane and the median plane of the 

 larvu, since the stained area may appear in any region of the 

 pluteus. Thus larva? have been found which differentiated both 

 as described by Driesch and by Boveri, and at every angle be- 

 tween. In spite of this fact Yon Ubisch believes that his results 



