150 Papers from the Marine Biological Laboratory at Tortugas. 



environment in which the same thing may always be expected to occur. 

 It is by a change from the optimum of one form to that of another that 

 we may expect to bring about change in structure. 



I stated in the beginning that the aim of this investigation is a 

 formulation of a statement of the conditions governing the resemblance 

 of the offspring to the parent. That aim has been at least partially 

 accomplished by the research. 



We have definite evidence that temperature and OH ion concen- 

 tration of -the sea -water are factors whose variation determines in part 

 this resemblance to one or the other parent. In other words, the struc- 

 ture of the sea-urchin pluteus from cross-fertilized eggs may be influ- 

 enced by the external environment to which the developing germ is 

 subjected during its growth. 



In a preliminary paper (igioa) I criticized Herbst's idea of nuclear 

 control. Our evidence, I believe, all points to a physical-chemical 

 control. This control is exerted on the germ-cell from its beginning. 

 This control is the complex of factors forming the environment of the 

 germ-cell during its growth in the body of the parent and (in forms in 

 which fertilization and development take place outside of the body) 

 the environment in which growth takes place. For the agent through 

 which this control is exerted, much evidence shows that we may look 

 to the nucleus. 



A further generalization from my observations would be premature. 

 We do not kno,w how permanent the changes in structure may be. 

 That knowledge ean be gained only by a study of later generations raised 

 from cross-fertilized eggs. 



SUMMARY. 



(1) The Toxopneustes $ X Hipponoe c^ and the reciprocal cross H ip- 

 ponoeQ X Toxopneustes (? were easily made after allowing the eggs to 

 stand in sea-water for some hours before fertilization. 



(2) In the embryos of both crosses made in ordinary sea-water, 

 which was alka-line, the Hipponoe influence showed a tendency to pre- 

 dominate. 



(3) In the embryos of both crosses made in sea-water of increased 

 alkalinity, there was evidence of an increase of Hipponoe influence. 



(4) In the embryos of both crosses made in sea-water of decreased 

 alkalinity, a tendency toward Toxopneustes dominance was evident. 



(5) The results thus show Hipponoe dominance in sea-water of a 

 higher OH ion concentration and Toxopneustes dominance in sea-water 

 of a low r er OH ion concentration. 



(6) I suggest that these variations in the alkalinity of the sea-w r ater, 

 which I have broiight about artificially, may correspond to normal 

 seasonal changes. 



(7) If this be true, the winter (paternal) embryos and the summer 

 (maternal) plutei of the combination Sph&rechinus X Strongylocentrotus 

 of other investigators had their origin in such normal seasonal changes 

 of OH ion concentration. 



