18 ROBERT W. HEGNER 



the binucleate members of family 150 (fig. 17, .4) had produced 

 a specimen with only one-half of a shell and with only one 

 nucleus (fig. 17, B). The later offspring of this parent were 

 entirely normal (fig. 17, C). Fifty descendants consisting of 

 thirty-five uninucleates, one empty shell, and fourteen binu- 

 cleates were reared from this uninucleate half-specimen, and 

 these were in every way similar to those reared from similar 

 pieces obtained by cutting operations. One uninucleate from 

 this line was bisected, and twenty-four descendants were ob- 

 tained from the nucleated portion. As in other similar experi- 

 ments to be described later, uninucleate offspring were at first 



A Be 



Fig. 17 Arcella dentata. Outline drawings showing the binucleate parent 

 (A) of the uninucleate half-specimen (5), and one of the binucleates (C) from the 

 line derived from this half-specimen. X 207. 



produced, but later, empty shells were formed and binucleates 

 appeared which finally reached a diameter and spine number 

 characteristic of the family 150. 



8. Microdissection experiments. What effect does the removal 

 of one nucleus and no cytoplasm have upon the specimen and 

 its progeny? In the experiments already described cytoplasmic 

 substances as well as nuclei were removed. It was found pos- 

 sible, however, to remove the nuclei alone by means of a Barber 

 microdissection apparatus.'- If a very" small portion of the edge 

 of the shell is broken away near where a nucleus is located 



- The writer is indebted for the use of this instrument to Dr. Warren Lewis, of 

 the Johns Hopkins Medical School. 



