ZOOLOGY: R. W. HEGNER 
283 
The third instance is the maturing of the cerebellum, represented by the 
completion of the Purkinje cells and the disappearance of the external granule 
layer. In the rat these events occur between birth and 20 days of age, Addison 
'11. Like events occur in the human cerebellum and are completed in man at 
nearly the equivalent age. When the cerebellum has so far matured, locomotor 
control is attained in both forms, and thus this series of histological adjust- 
ments and locomotor control are accomplished at nearly equivalent ages in 
both the rat and man. 
Finally, Dr. Sugita ('17) has just completed a study of the growth in thick- 
ness of the cerebral cortex of the rat, and the graph A in chart 3 shows that 
the mature thickness is nearly attained at the age of 20 days. There are at 
present no systematic studies on this point for man, but two incidental obser- 
vations, entered as heavy dots, agree with the inference that at 15 months, 
the equivalent age, a like degree of completeness is reached by the human 
cerebral cortex, and therefore that only slight growth in the thickness of the 
human cortex is to be expected after this age. 
There are therefore five prime events in the growth history of the nervous 
system of the rat, namely: (1) increase in total weight; (2) decrease in the 
percentage of water; (3) accumulation of myelin; (4) maturing of the cerebel- 
lum; (5) the attainment of the mature thickness of the cerebral cortex, all of 
which takes place at ages equivalent, or nearly equivalent, to those at which 
they occur in man. 
It appears then that by the use of equivalent ages we have a satisfactory 
method for making a cross reference between the rat and man, and because 
the growth changes are similar in both forms, the rat may be used for further 
studies on the growth of the nervous system with the assurance that the 
results so obtained can be carried over to man. 
Addison, William H. F., Wistar Inst., Philadelphia, J. Comp. Neur., 21, 1911 (459-481). 
Donaldson, H. H., Ibid., 26, (1916), (443-451); these Proceedings, 2, 1916, (350-356).. 
Sugita, Naoki, /. Comp. Neur., 28, 1917, (511-591). 
VARIATION AND HEREDITY DURING THE VEGETATIVE 
REPRODUCTION OF ARCELLA DENT AT A 
By R. W. Hegner 
Zoological Laboratory, Johns Hopkins University 
Communicated by H. S. Jennings, June 15, 1918 
The conclusions of several investigators, that the genotype is constant in 
organisms that are multiplying by fission, have recently been put in question 
by the work of Middleton 1 (1915) on Stylonychia and by Jennings 2 (1916) on 
Difflugia. Middleton obtained two lines of Stylonychia from a single specimen 
