310 PROCEEDINGS OF THE ACADEMY OF [April, 



segments begin to condense and a longitudinal split is seen in them 

 (fig. 81). Though the number of segments at this time could not be 

 counted, yet it was plainly the reduced number, and this reduction 

 must have taken place during the last few stages when, because of 

 the deeper staining and greater massing of the chromatin, it was 

 impossible to follow the steps by which this pseudo-reduction occurred. 

 The ring chromosomes of the prophase are seen to be formed by the 

 coming together of the split ends and the gradual condensation of the 

 chromatin mass: this concentration is complete in sections, and 

 dumbbell-shaped chromosomes and occasionally tetrads are formed, 

 which are connected by linin threads (figs. 83-85). At this stage of 

 prophase, when growth has reached its greatest extent, the cell is 

 several times as large as in the beginning of the growth period, and the 

 nucleus, which has increased in size with the cell, is usually eccentrically 

 placed. At this time chromatin granules similar to those described 

 for C. promethea are found in the cell cytoplasm and are traceable 

 throughout all succeeding steps. 



A metaphase of the first maturation division shows thirty chromo- 

 somes (fig. 87) which are symmetrically placed; their division is equal 

 and reducing. The chromosomes of the first polar plate are too con- 

 densed to count and quickly pass through the prophase of the sec< nd 

 maturation. The centrosomes move so that the second division is a1 

 right angles to the first and the chromosomes quickly arrange then - 

 selves for the second division, in which, as in the first, there is an equal 

 division of the chromatin: an equatorial plate of the second spindle 

 shows thirty chromosomes, smaller than in the first and always single 

 (fig. 92). The distinct zwischenkorper and a number of the chromatin 

 granules mentioned above are seen during telophase both in the 

 cytoplasm and upon the mantle fibres (Pi. XXV, fig. 98). The 

 development of the spermatid shows no new features, but corresponds 

 very closely to that described for C. promethea. 



Centrosomes are seen throughout all stages: in the resting sper- 

 matocyte they appear as small paired structures closely pressed to 

 the cell membrane and with the short flagella? extending into the 

 lumen (fig. 79); as development proceeds these centrosomes move 

 nearer to the nucleus and migrate to opposite sides of the cell in prepara- 

 tion for the first division, and in late prophase and anaphase stages 

 they appear as dumbbells and are so conspicuous by their size and 

 prominence that they might almost be taken for very small chromo- 

 somes. Astral rays are well developed and, so far as could be deter- 

 mined, these grow out directly from the centrosomes. 



