262 THOS. H. MONTGOMERY. 



i 



nal split appears later and then along the axis of each arm (Figs. 

 3-6). What then is the correct interpretation of each chromo- 

 some loop? Each U or V is, first of all, a bivalent chromo- 

 some, since they occur in half the number of the chromosomes 

 of the spermatogonia. Second, each arm of one of these bivalent 

 chromosomes represents one of the univalent chromosomes of the 

 spermatogonia. In the case of a bivalent chromosome of the 

 form of a U or V, the two univalent chromosomes are connected 

 together at the angle, that is one end of one chromosome is 

 joined to one end of another chromosome ; these points of union 

 are marked in the Figs. 2, 3, 5 and 6 by the letter A". Just at 

 this point of junction can be seen in many cases, though not so 

 clearly as in Peripatns, a broad connecting linin thread, as in 

 the chromosomes of Fig. 5. When a bivalent chromosome is 

 an elongate ring, as in the case of the left-hand one of Fig. 3, 

 then both ends of both univalent chromosomes are connected. 

 The bivalent chromosomes of Desmognathus and Plcthodon thus 

 represent each two univalent chromosomes joined end to end, 

 and the space between the two arms of a bivalent chromosome 

 is the space between two univalent chromosomes, whether this 

 space be bounded by a chromosome of the form of a U, a V, or an 

 O. This is the space marked in the figures by the letter .r, and, 

 as the present observations show, does not represent a longitudi- 

 nal split since it is widest at the earliest stages of the prophase. 

 The longitudinal splitting of the chromosomes is shown in its 

 commencement in Fig. 3, where certain microsome groups are 

 splitting ahead of others. Fig. 5 shows the stage where this 

 split is most pronounced. Fig. 4 shows the split on cross- 

 sectioned chromosomes. It is still apparent in some of the 

 chromosomes of Fig. 7, but in the equatorial plate stage (Fig. 8) 

 is hidden, to reappear at the anaphase as a longitudinal split of 

 each of the daughter (univalent) chromosomes. This tempo- 

 rary disappearance of the longitudinal split during the equatorial 

 plate stage of the first maturation mitosis has been noted in 

 various objects, and I have described it in detail for Pcripatns 

 and the Heiniptcra, as has Korschelt for Ophryotrocha. Along 

 the plane of this split the daughter chromosomes divide in the 

 second maturation mitosis, so that the latter is an equation 

 division. 



