ORIGINS OF EMBRYONIC PATTERNS 697 



very general belief that the specific characteristics of protoplasms result 

 chiefly from their proteins and to the conception of protein molecules as 

 possessing definite spatial patterns, polarity, and symmetry or asym- 

 metry, he suggests that embryonic developmental pattern results from 

 the configuration of protein molecules. In consequence of their bipolar 

 character they tend to orient in the cell, "possibly with respect to the 

 point of attachment in the ovary." Different chemical properties at the 

 two poles bring about different reactions; and the resulting substances are 

 carried electrophoretically toward opposite poles of the cell, forming two 

 opposed material gradients, each decreasing in concentration from one 

 pole to the other. Differences in concentration along these gradients, to- 

 gether with substances of nuclear origin, initiate new reactions locally 

 and so begin the progressive localization and complication of develop- 

 ment. Symmetry and asymmetry are supposedly further expressions of 

 the molecular configuration 



This hypothesis admits the possible relation of developmental pattern 

 to environment in suggesting that the molecular orientation may occur in 

 relation to attachment of the oocyte, but it does not suggest how the 

 orientation is brought about. If orientation does occur in this way, must 

 it not be because a difference in condition between point of attachment 

 and other regions produces a differential of some sort in the cell? If this 

 be granted, it is probable, since the cell consists of living, metabolizing 

 protoplasm, that this differential will involve a differential or gradient in 

 metabolism, rate of living, or whatever we may call the continuous physi- 

 cochemical change constituting life. In short, this hypothesis of Harri- 

 son's does not exclude, but seems to require, a gradient involving the es- 

 sential activities of the protoplasm to bring about the molecular orienta- 

 tion. It is perhaps questionable whether molecular orientation ever occurs 

 spontaneously, that is, entirely without any relation to factors outside 

 the molecules concerned. If such a gradient in the oocyte is a factor in 

 molecular orientation, it, not the orientation, is the primary axiate pat- 

 tern. 



Origin of symmetry and asymmetry in terms of this hypothesis raises 

 further questions. If the molecules possess a dorsiventral and bilateral or 

 asymmetric pattern and different substances are produced on different 

 sides, they, like the polar substances, must be transported to opposite 

 sides of the cell, dorsally and ventrally or laterally; but how? Independ- 

 ent cataphoresis in two opposite directions along two or three different 

 axes does not appear possible. All such hypotheses require postulation 



