iS Papers from the Marine Biological Laboratory at 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 Sinety during this same stage as the " chromosomes speciales." 

 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 ) ' 17 ordinary -f- 2 accessory chro- 



