52 LIBBIE H. HYMAN. 



The existence in the development of vertebrates it is now 

 known to occur in the frog and chick as well as in teleosts of two 

 regions of high activity, one at each end of the axis, is correlated 

 with the fact that vertebrates are segmented animals. This mode 

 of development is common, as far as our investigations go, to all 

 segmented animals. This " double gradient," as we call it, was 

 first discovered in the annelids (Hyman, '16, and Child, '17). 

 It appears in an early stage of development in annelids and per- 

 sists throughout life in all of them, so far as tested. It also 

 appears, as we have seen, in the vertebrate embryo and persists 

 as long as the posterior end continues to develop and elongate. 

 In the annelids where new segments form continually throughout 

 life, the posterior region of high susceptibility is permanent and 

 never brought under complete control of the anterior end ; but in 

 the vertebrates it eventually dies away and segment formation 

 thereupon ceases. The posterior region of activity is therefore 

 correlated with the process of segment formation. There can be 

 little doubt that segments represent incomplete individuals. As 

 Child ('150) has shown, the anterior end of an organismic axis 

 is dominant over a certain length of the axis ; beyond this level 

 physiological isolation occurs, the metabolic activity increases, and 

 new individuals arise. A similar process is at the basis of seg- 

 mentation. The posterior growing region of the embryos of 

 segmented animals has escaped from the control of the anterior 

 end ; it is physiologically isolated, develops a high metabolic rate 

 and proceeds to the formation of new but incomplete individuals, 

 that is, segments. This process of segment formation will con- 

 tinue indefinitely if the anterior end fails to regain control of the 

 entire length of the axis as in the annelids but will cease when 

 this occurs as in the vertebrates and probably arthropods. These 

 matters have been discussed at greater length by Child ('17) 

 and Bellamy ('19). 



IV. RESPIRATORY RATE DURING THE DEVELOPMENT OF 



FUNDULUS. 



I have made some measurements of the rate of oxygen con- 

 sumption and the rate of carbon-dioxide output during the de- 



