18 Papers from the Marine Biological Laboratory ai Tortugas. 
gonia a deep-staining chromatin nucleolus (accessory chromosome) appears 
in the almost achromatic nuclear reticulum, of sharp contour and usually 
closely applied to the nuclear wall (fig. 13). Thus the accessory chromo- 
some first appears as a definite characteristic nuclear structure in the resting 
stage of the first order of secondary spermatogonia. It answers to the 
various morphological and microchemical tests for a chromosome. There 
was nothing corresponding to this body in the resting stage of the primary 
spermatogonia, nor yet in the late telophase of the final mitosis. Since the 
number of chromosomes of the later spermatogonial cells remains the same 
as that of the primary spermatogonia (35), the accessory represents prob- 
ably a specifically modified metabolic phase of an ordinary chromosome that 
had passed into the reticular stage in the telophase and returned to the com- 
pact stage much in advance of its fellows. At the next mitosis it passes 
. without visible change into the equatorial plate with the chromosomes that 
arise from the segmented spireme of the prophase, and its presence there 
does not alter the constant count of 35 chromosomes for the unreduced 
number. 
During the prophase of the ensuing mitosis the chromatin passes through 
the fine, coarse, and segmented spireme stages (figs.14 and 15). Frequently 
the accessory chromosome gives indication of a bipartite structure presaging 
its later division in metakinesis. The long, mossy, deep-staining segments 
of the prophase shorten, condense and split longitudinally into pairs of short, 
slender rods, among which the accessory chromosome has become unrecog- 
nizable. These pairs of rods unite and are assembled in the equatorial 
plate as very chromatic rod-shaped bodies of variable size. Usually one 
among the number is typically U-shaped (fig. 17). The chromosome count 
is constantly 35 (figs.17 and 18). Occasionally an equatorial plate showing 
as many as five U-shaped chromosomes similar in size and shape (fig. 20) 
is found. Figure 21 shows four spindles with the chromosomes at various 
stages of metakinesis. There is no mark or sign by which the accessory 
chromosome can be recognized at this stage. In the ensuing telophase 
stages (figs. 22 and 23) one pair of chromosomes always lags somewhat 
behind its fellows. This pair of chromosomes corresponds to those desig- 
nated by de Sinéty during this same stage as the “ chromosomes spéciales.” 
However, while there is always one lagging pair, there may be several (fig. 
23), and this fact renders the precise determination in all cases of the acces- 
sory chromosome impossible. 
Figure 24 shows a twin spindle with the chromosomes at telophase. 
Again there is a lagging pair. This figure is undoubtedly the result of an 
amitotic nuclear division in an early primary spermatogonial mitosis result- 
ing in a binucleate cell. The daughter-cells of such a doubly-endowed 
mother-cell probably give rise to the primary spermatocytes with double the 
number of chromosomes (36 in this case, 2 X 17 ordinary + 2 accessory chro- 
