52 CHARLES W. METZ 



TYPES IV AND V 



Chromosome groups found respectively in Species C 

 and D. tripunctata 



Like Types II and III, these two differ from one another mainly 

 in the presence and absence of the m-chromosomes. Otherwise 

 they are essentially alike, and may be considered together. Fig- 

 m-es 18 and 19 show early metaphases of Type IV taken from 

 aceto-carmine smear preparations. Figure 20 shows the haploid 

 nmnber of the same, in the second spermatocyte division (equa- 

 tion division), from a section. In figures 21 to 26 are shown the 

 diploid chromosomes of Type V, taken from sections. The first 

 two figures represent metaphases — 21 a polar view, and 22 a 

 somewhat diagonal view. They differ in proportions from the 

 figures of Type IV because of the contraction incident to fixing, 

 embedding, etc., but nevertheless show their general similarity 

 to the other type; (compare figs. 18 and 22). Figure 23, a late 

 prophase of Type V, represents an earlier stage, before the auto- 

 somes have fully contracted. Figure 24 is an early prophase, 

 included here to show the precocious contraction of the sex- 

 chromosomes, and the very intimate pairing or conjugation of the 

 autosomes. In figures 25 and 26 are shown the two poles of an 

 anaphase, indicating the behavior of the three different kinds of 

 chromosomes at this stage. 



These two types (IV and V) are less easily analyzed than the 

 previous ones, but the evidence clearly indicates that they are, in 

 a way, intermediate between certain of the latter. Comparing 

 Type IV with Type I, as indicated in diagrams 4 and 1, it would 

 seem that two of the large autosomes of Type I have undergone 

 partition, giving rise to four small autosomes, while the other 

 large pair has remained intact. By a similar partition of the 

 latter a type like number II would be formed. From Type IV 

 one like Type V could be derived by loss of the m-chromosome 

 pair, as assxmied in the case of Type III, either by degeneration, 

 or by fusion with another pair. These various steps are indicated 

 schematically in diagrams 6 to 11. 



