No. 3.] THALASSEMA AND ZIRPHAEA. 609 



The crosses are to be derived from the double rods by a 

 swelling or looping up of the halves in the middle. The begin- 

 ning of the process is represented by the previously described 

 thick rod with central bead-like swelling (Fig. II, /). In a 

 slightly later stage, a central hollow appears which, separating the 

 swelling into two knobs, shows the rod to be double (o, ti). In 

 the form figured at the rod is greatly curved, but it may some- 

 times be straight, when a dagger-shaped figure results {0, u), 

 often with a clear central split. In this respect also it differs 

 from Klinckowstrom's figure of a dagger-like chromosome in 

 ProstheccracHS. The metaphase crosses, with curved lateral 

 arms, may have resulted from rods curved like i or v. A further 

 increase in size of the lateral swellings produces a cross (g) 

 quite similar to those of the equatorial plate. ^ 



The chromosomes now being taken into the spindle (PI. 

 XXXI, Figs. 12, 13), it becomes of importance to determine 

 whether the arms formed by the mid-swelling of the halves of 

 the rod become the polar or the lateral arms. In the former 

 case the axis of the double rod (which is that of the original 

 spireme segment), being coincident with the equatorial plane, 

 the first division would be an equation division. If, on the 

 other hand, they become the lateral arms, the reverse would 

 hold, and the resulting division would be a reducing division. 

 The former I believe to be the correct alternative. For, in 

 the prophases, the lateral swelling seems considerably shorter 

 than the two other arms of the cross, while in early metaphase, 

 the equatorial arms are often quite long, and the polar ones 



bility suggested by Wilcox ('95), that in one and the same spindle some chromo- 

 somes undergo a reducing, and others an equation division. The occurrence of 

 varied prophase chromosome figures, all of which subsequently develop into a 

 uniform type of metaphase figure, recalls Calkins's ('97) results in the fern. In 

 Thalassema, however, the end result is not a four-sphered tetrad (as in the fern), 

 but a ring variously looped and, as we have seen, a careful comparison of all 

 intermediate stages shows that the interpretation given by Calkins of the crosses 

 in the fern does not hold in Thalassema. 



1 Considerable variation exists in the degree of approximation of the halves of 

 these figures, and hence in the clearness with which the central split appears. 

 Rarely it appears with great clearness, but more often it can be satisfactorily 

 detected only by the very best optical means (e.g., Zeiss, Apoch. 1.5 mm. oculars 

 6-12, with most powerful light), while sometimes even these fail to bring it out, 

 and the arms then appear perfectly solid. 



