36 Henriksen, A Functional view of Development. 



rather than form, heredity is strictly speaking, a physiological 

 and not a morphological problem." 



From the above we can see that the study of regeneration 

 leads to a similar conclusion to those expressed in this paper, 

 namely, that the problem of development and heredity is not 

 primarily a structural but a functional one, and this as I have 

 tried to show may be referred to the egg just as well as to the 

 embryological development. 



We have still two important factors to discuss in Order to 

 understand development, namely, environment and nutrition. I shall 

 here but quote a few classical examples and leave a fuller discus- 

 sion for some future time. Prof. J. Loeb bas shown that regene- 

 ration and growth in many aquatic animals depends to a great 

 extent on the amount of water present in the cells. By bringing 

 hydroids in more diluted sea-water than that in which they usually 

 live, the rate of growth is increased. Similar results were gained 

 by similar methods on developing eggs of sea urchins, star fishes, 

 arthropods and various fishes. That the amount of water and the 

 intracellular pressure in these experiments varied with the concen- 

 tration could be seen from the form of cleavage spheres. In nor- 

 mal sea-water which was a little diluted by the addition of 10 — 20 

 percent of fresh water, the two first cleavages were merely hemis- 

 pheres. In sea-water of higher concentration the first two cleavage- 

 spheres became ellipsoidal in shape approaching more the sphere 

 as the concentration was increased. When more than two grams 

 of sodium cloride was added to 100 c. c. of sea-water in a few 

 hours plasmolysis took place and the surface of the protoplasm 

 began to shrink irregularly. But by bringing the eggs back into 

 normal sea-water the normal form was restored in a few minutes. 

 Exactly similar results were gained when the temperature was 

 raised and lowered. 



When we consider that the sea-water differs in concentration 

 near the shore among the rocks, with that of other places as well 

 as does the temperature at various depths, etc., we see at once 

 the importance of producing the exact conditions in density, tem- 

 perature, pressure, and Light in experimental embryology under 

 which these animals naturally develop in order to produce normally 

 developed embryos. And still more are the external influences 

 emphasized by Loeb's experiments on artifical production of 

 parthenogenesis in sea urchin eggs by adding a small amount of 

 magnesium cloride to the sea-water. 



Still more striking changes are brought about by change of 

 food. I shall here but give one illustration. 



If an embryo of a sea-urchin be made to develop in sea water 

 containing no lime salts the larya fails to develop the calcareous 

 skeleton. Such a result has its foundation in the first principle 

 of physiology, namely, that no organism can produce chemical 

 elements in order to produce the calcareous skeleton. The elements 

 which go to make up this skeleton must be taken in as food and 



