1 66 Papers from the Marine Biological Laboratory at Tortugas. 



the nucleus, frequently, but not invariably, next the idiosome. The re- 

 mainder of the nuclear space remains empty, except for a few delicate linin 

 threads, more abundant in the vicinity of the mass. All about the idiosome 

 are pale spherical mitochondria, now apparently present for the first time; 

 but no clue appears as to their origin. 



The fact that the synizesis nucleus is not appreciably larger than that 

 of the resting stage shows that synizesis is not due to mechanical factors 

 consequent upon a great increase in nuclear sap. On the contrary, this 

 phenomenon is unquestionably the result of the specific activity of the 

 chromatic nuclear thread. Its frequently polar position may be due to 

 idiosome influence. 



In the bouquet stage the simple threads emerge from the synizesis mass 

 in pairs (fig. 5) and unite side by side (parasynapsis) . The nucleus is still 

 of approximately the same size; and again the chromatin threads give evi- 

 dence of a specific activity. The process continues until the previously 

 empty nucleus is filled with double threads crossing in every direction 

 throughout the nucleus (fig. 6). Subsequently the nucleus enlarges some- 

 what, and the double threads shorten and thicken (fig. 7). The beginning 

 of division of the idiosome marks this as the early prophase. The bivalent 

 chromosomes appear more chromatic and knobbed at the ends, an early 

 indication of tetrad formation. Figure 8 illustrates a later prophase stage, 

 in which the chromosomes have become still shorter, uniformly chromatic, 

 and more compact, a number appearing as complete tetrads. 



The point of special interest is that nowhere throughout these early 

 phases has the slightest evidence of an accessory chromosome appeared. 

 No evidence would be expected to appear subsequently, and this is actually 

 the case, as shown in figures 9 to 17. At metaphase the chromosomes are 

 .all arranged in a very compact equatorial plate (figs. 9 and 10). No 

 •chromosome seems marked by peculiar behavior or uncommon size. An 

 ;accurate count seems impossible. Innumerable division figures appear, 

 Ibut the chromosomes are so closely compacted as to preclude definition of 

 limits. A most careful study warrants only the statement that the number 

 of chromosomes is somewhere around 24. 



A resting stage ensues, but without chromosome-nucleolus, in contra- 

 distinction to the usual condition in forms with an accessory chromosome, 

 or without even a plasmosome. The division figures of the second mitosis 

 are smaller, somewhat less compact, and seem to consist of fewer chromo- 

 somes. We may be dealing here, as in the opossum, with a second pairing, 

 or a hemioid group (Jordan, 191 1). The smaller size of the chromosomes, 

 however, makes this a little doubtful. 



Figure 14 illustrates a resting spermatid, with conspicuous idiosome. 

 At the next step in the metamorphosis (fig. 15) the cell has elongated, the 

 nucleus has become more dense and in consequence smaller, and the idio- 

 some has given place to the archoplasmic sphere. The latter contains 

 centrally a chromatic granule, presumably the centrosome (fig. 15). The 



