176 PROCEEDINGS OF THE ACADEMY OF [Feb., 



plasm). During this process there appears to be a continuous Hnin 

 spirem, but no continuous chromatin spircm; the bivalent chromo- 

 somes in the form of V's or U's are arranged upon the linin thread so 

 that their free ends are toward the distal nuclear pole. The distal ends 

 of each V are rarely contiguous with those of a neighboring one, though 

 that is sometimes the case, but, as the figures show, they are usually 

 slightly separated. Between the distal end of one univalent component 

 of a V and the corresponding end of a similar component of another V 

 can be seen, whenever these structures lie in the same plane, a connect- 

 ing linin thread. These phenomena are so similar to those in Peripatus 

 that I think it unnecessary to describe them all in detail again, and 

 refer to the very detailed account of the Peripatus paper. A pole view 

 of a nucleus in the synapsis stage shows the optical cross-section of 

 twenty-six chromatin loops, and lateral views demonstrate the presence 

 of thirteen V's. Each V, therefore, corresponds to two univalent 

 chromosomes of the spermatogonia ; it is a bivalent structure in which 

 each arm represents one chromosome, and has been formed by the 

 conjugation of two end to end and not by transverse scission of a con- 

 tinuous chromatin spirem. Where the ends of two conjugated univa- 

 lent chromosomes come together (the points marked x in these and the 

 subsequent figures) is frequently found a slight notch or break, which 

 is a connecting band of linin — corresponding to the central linin band 

 in Peripatus. That the space between the two arms of a V is not a 

 longitudinal split is indubitable, for stages like that of fig. 42 show 

 previously separated chromosomes coming together. The longitudinal 

 split appears in the long axis of each univalent chromosome (figs. 

 42-44), and proceeds latest to their distal ends (those directed toward 

 the distal pole of the nucleus). No earlier longitudinal split occurs, and 

 no later one. A nuclear membrane appears first at the close of the 

 synapsis. 



The behavior of the heterochromosomes can be followed with equal 

 facility and certainty. All through the growth period they preserve 

 their smooth contours, compact structure and strong affinity for chro- 

 matin stains. The two univalent heterochromosomes (N. 2, fig. 42) 

 come together and conjugate side to side (figs. 4.3, 44), though their ends 

 directed toward the distal nuclear pole are in closer touch than their 

 opposite ends, in contrast to the behavior of the other chromosomes. 

 So is produced a bivalent heterochromosome, throughout the growth 

 period placed against the distal pole of the nucleus, consisting of two 

 univalent chromosomes lying parallel or more usiially in the form of a 

 much narrowed V. It does not increase in size in the following stages, 



