GERM CELLS IN PEDICELLINA AMERICANA 33 



sponding or homologous maternal and paternal chromosomes 

 occurs ; for, as Sutton has pointed out, this phenomenon, if it 

 really occurs, gives the basis for a probable explanation of the 

 Mendelian phenomena of heredity. 



In Pedicellina, the phenomena are such as to give no evidence 

 as to whether such conjugation of paternal and maternal ele- 

 ments occurs, yet the facts give good ground for supporting 

 the more general conclusion that a union of chromosomes, two 

 by two, takes place at this period. In this way are produced 

 the bivalents of the long growth period, giving almost decisive 

 proof of the validity of Hacker's early interpretation of the 

 chromosomes in the post-synaptic stages. If this be granted 

 there can be no escape from the conclusion, in the light of the 

 succeeding processes, that one of the maturation divisions is 

 transverse, separating pairs of identical (sister) chromosomes. 

 It is now well to review in greater detail, the various phases of 

 the development of both the ^^^ and the sperm cells to see 

 more clearly the grounds for this accordance with the results of 

 the above. 



{A) "■ The Individuality of the Chromosomes.'' 



This hypothesis has recently received strong confirmation at 

 the hands of students of insect spermatogenesis (McClung, 

 Montgomery, Sutton, etc.). Not only have these authors ob- 

 served constant size differences in the chromosomes, but in 

 the case of certain peculiar elements, the so-called accessory 

 chromosomes, the persistence of the structures as such has 

 been traced throughout several generations of spermatogonia, 

 the two spermatocyte divisions and finally into the spermatids. 

 This is fully in harmony with the experimental results of 

 Boveri, who, in his remarkable paper on '' Multi -polar Mitoses " 

 ('02) was able to conclude " dass nur eine bestimmte Kombina- 

 tion von Chromosomen, wahrscheinlich nur die Gesamtheit der 

 in jedem Vorkern enthaltenen, das ganze Wesen der Organismen- 

 form, soweit dasselbe vom Kern aus bestimmt wird, reprasen- 

 tiert." an,d " dass nicht eine bestimmte Znhl, sondern eine be- 

 stimmte Kombination von Chromosomen zur norma- 



