BLACKMAN: SPERMATOGENESIS OF THE MYRIAPODS. <0 



some has increased enormously in size, much out of proportion to the 

 rest of the nucleus. 



3. At this stage the appearance of the nucleus corresponds very 

 closely to that of the germinal vesicle in the egg-cell. The cytoplasm 

 has greatly increased, and large masses of yolk substance have ap- 

 peared, surrounding the nucleus. On account of these characteristics 

 this stage has been called the pseudo-germinal-vesicle stage. 



4. The chromatin appears after the j)seudo-germinal- vesicle stage 

 in the form of diffuse masses of chromatin, which quickly take on the 

 cruciform shape characteristic of the typical insect tetrad. 



5. At the time of the formation of the tetrads, the accesisory chro- 

 mosome has decreased much in size and its outline has become irregu- 

 lar. Later it again becomes spherical, and at this period is of about 

 the size of a chromosome in the metaphase. 



6. Soon after the formation of the tetrads, the centrosome appears 

 in the mass of deutoplasm as a dumb-bell-shaped bodj'. It moves to- 

 ward the nucleus, separates, astral rays are developed, and the centro- 

 somes move apart upon the membrane. When they have reached 

 points about 100 degrees apart the nuclear membrane disintegrates, 

 and the chromosomes are drawn into the equatorial plate. That part 

 of the membrane over which the centrosomes have not passed disap- 

 X)ears first, the rest persisting for some time. 



7. The accessory chromosome, freed of its surplus chromatin, can 

 be distinguished up to the disappearance of the nuclear membrane, 

 when it is drawn into the plate with the other chromosomes. It has 

 previously shown no sign of division. 



8. The short spindle thus formed lengthens, and the chromosomes 

 are drawn more into the equatorial plate. Astral rays may be dis- 

 tinctly seen connecting the centrosomes and the cell membrane. 

 The centrosomes move apart again to a point a short distance from 

 the cell-wall. The mantle fibers no longer converge toward the cen- 

 trosomes, but toward the point (the apical point) occupied by them 

 before their last migration. Between the apical points and the centro- 

 somes the mantle fibers are drawn out into parallel threads. At this 

 time the centrosomes have changed their form to that of a cone, with 

 their apices directed toward the point of the spindle. 



9. By the contraction of the astral rays the centrosomes are drawn 

 to the cell-wall, where they are flattened out into hemispherical bodies. 

 Synchronously the mantle fibers contract, the chromosomes are sep- 

 arated, and the two daughter groups are drawn toward the poles and 

 take up their final jjositions at the apical point. 



10. The second spermatocytes can be distinguished from the first 

 only by the shape and size of the chromo.somes. 



11. Soon after the formation of the membrane, after the last ma- 



