PHYSIOLOGICAL CHARACTERISTICS OF AXIATE PATTERNS 129 



creases from the head posteriorly for a certain distance, and that the earh- 

 er stages of segment development in the posterior region have a higher 

 rate than later stages at more anterior levels is indicated by a large body of 

 evidence, even though there is not complete agreement. 



EMBRYONIC AND LARVAL DEVELOPMENT OF ECHINODERMS 



Embryonic development of echinoderms is one of the classic materials 

 of developmental physiology. It has been the object of a great number of 

 experimental studies dealing with many special problems, but all more or 

 less directly concerned in one way or another with the problem of develop- 

 mental pattern. Among the many investigations concerned with this 

 material, the concept of a gradient or gradients of some kind as essential 

 factors of axiate pattern has appeared in various forms. Probably the 

 first suggestion that the polar axis is represented by a gradient was ad- 

 vanced by Boveri (1901). The earlier experimental studies of Runn- 

 strom (1914, 191 5) on echinoderm development led him to the view that 

 the axiate pattern of sea-urchin development is based on systems of layers 

 or concentration gradients of substances. Further development of this 

 hypothesis by Runnstrom and his co-workers, particularly Horstadius 

 and Lindahl, will be discussed as occasion arises. A hypothesis of gradient 

 pattern in sea-urchin development in terms of the colloid substrate has 

 been advanced by von Ubisch (1936, 1938). Observations on differential 

 susceptibility, as indicated both by differential death and by differential 

 modification of development, on differential reduction of potassium per- 

 manganate and later of dyes and in the indophenol blue reaction, in other 

 forms as well as in echinoderms, led Child to a hypothesis of axiate pattern 

 in terms of gradients in which differences in rate of metabolism were re- 

 garded as essential factors and probably the most important in early de- 

 velopment.^^ 



In this chapter only some of the more direct evidence on which these 

 hypotheses are based will be dealt with.-^ 



'•t Child, 1913a, d; 1914/; 1915a; 1916a, d; i928(f; 1936a, b. 



^5 In order to avoid possible confusion, certain axial relations in echinoderm development 

 and the terms used in describing them are noted. The terms "animal" and "vegetal" or "vege- 

 tative," dating from the earlier days of embryology but still widely used for eggs and early 

 embryonic stages generally, have reference to the more or less marked segregation of proto- 

 plasm and yolk and the difference in developmental activity in opposite polar regions of the 

 eggs of many animals. The more active protoplasmic pole where polar bodies are usually 

 formed is the animal pole. The application of these terms to echinoderm development in 

 which polar differences are slight in early stages is largely conventional. In the following pages 



