278 Chemical Differentiation of the Brain 



are found to be in corresponding states at the -&V and the yiur part 

 of the total life cycles. Had the relation, in the form stated above, 

 held, these fractions should have been identical. It is only fair 

 to add, however, that in view of the absence of precise information 

 concerning the pig and in view of the fact that the early days of 

 gestation are used for cell division accompanied by only slight differ- 

 entiation, too much stress should not be laid on the relation here 

 given. 



On the other hand, instead of taking the end of life as the fixed 

 point of 'our calculations, we may consider the time when motor 

 control is obtained to indicate closely corresponding states of the 

 central nervous system. In the rat, motor control is obtained at 

 42 days from conception, and in the pig at 125 days, that is, at 

 birth. If the law of corresponding states is correct, the nervous 

 system of these two animals should be in corresponding conditions 

 at the same fractions, either J, J, or J of these periods. This is 

 found to be the case, for the rat is born after 21 days' gestation. 

 This would be just half way between the two fixed points of concep- 

 tion and time of gaining motor control and this corresponds in the 

 pig to just one-half of its gestation period or about 62 days, which 

 is the age of the 100 mm. pig. 



It was actually found, both chemically and anatomically, that 

 the nervous systems of these two animals were in the same state 

 of development at these respective periods and it appears from these 

 observations that Donaldson's law may hold, if put in the form: 

 the nervous systems of mammals are in the same physiological 

 state at equal fractions of their total periods of development. 



In conclusion it gives me great pleasure to thank Dr. H. H. Don- 

 aldson and Prof. A. P. Mathews for their many suggestions in 

 connection with this problem and for their aid in getting this paper 

 ready for publication. The problem itself was suggested to me by 

 my brother and forms the first of a series of papers which are to 

 follow from time to time, on the chemical differentiation of the 

 central nervous system, and on which he was engaged at the time 

 of his death. The work was carried out in the Laboratory of Bio- 

 chemistry and Pharmacology of the University of Chicago and 

 was aided by h grant from the Wistar Institute of Anatomy and 

 Biology, Philadelphia. 



