4 ROBERT W. HEGNER 



c. Methods 



The methods of rearing the specimens recorded in this paper 

 are the same as those previously described (Hegner, '19). The 

 operations were for the most part simple. The specimens were 

 first drawn with a camera lucida; they were then cut in pieces 

 with a small, sharp scalpel, and the positions of the cuts were 

 indicated on the sketch. In one set of experiments the nucleus 

 only was dissected out with the aid of a Barber microdissection 

 apparatus. Parts of specimens or those from which nuclei had 

 been removed were cultivated as were the entire animals. 



2. EXPERIMENTS ON ARCELLA DENTATA 

 a. Experiments on binucleate members of family 150 



1. Introduction. The progenitor of this family (fig. 1) was 

 taken from a pond on the campus of the Johns Hopkins Uni- 

 versity at Homewood, Baltimore, on December 27, 1917. Its 

 spines consisted of almost indistinguishable ridges and could not 

 be counted. In diameter it measured 34 units of 4.3;u each. 

 Four immediate offspring were obtained from this specimen, 

 all of which exhibited well-defined spines, showing that the 

 absence of spines in the parent was probablj^ due to some envi- 

 ronmental factor. The fourth offspring was represented by an 

 empty shell; the other three possessed 13, 14, and 14 spines, 

 respectively; one was 34 units and the other two were 35 units 

 in diameter (fig. 2). 



2. Results of removing part of the shell and part of the cytoplasm. 

 The first experiment was designed to answer the following ques- 

 tions. If part of the shell is removed, is a new^ part regenerated? 

 What influence on a specimen and its descendants has the re- 

 moval of part of the cytoplasm and part of the chromidia? The 

 first offspring was operated on as indicated in figure 3. Part of 

 the shell and some of the cytoplasm were removed. No regen- 

 eration of the shell occurred. The first offspring produced by 

 this specimen after the operation was smaller than the parent 

 (15-31 in fig. 2), this decrease in size being due probably to the 



