1910.] NATURAL SCIENCES OF PHILADELPHIA. 305 



e. Condensation now takes place into dumbbells and tetrads and the 

 longitudinal and transverse planes of division can be clearly made out. 

 4. Maturation Divisions. — The chromosomes formed by the conden- 

 sation of rings now become arranged upon the fibres so that a side view 

 of the spindle during metaphase shows the chromosomes as symmetri- 

 cal dyads which are so placed that the first division, which is always 

 equal, may be seen to be transverse, and, if conjugation has taken 

 place as I believe by an end-to-end union of the chromosomes, division 

 is reducing (PL XXIII, figs. 42, 43). An equatorial plate during meta- 

 phase shows nineteen chromosomes (PL XXIII. fig. 40). While the 

 chromosomes are in metaphase the centrosomes divide and each becomes 

 enclosed by a small centrosphere ; as division proceeds the astral rays 

 increase in length until they seem to press against the wall of the cell, 

 a constriction is seen in the cytoplasm and the zwischenkorper is formed 

 at this point (fig. 44). During late anaphase the chromosomes become 

 so crowded as to lose their separate outlines (fig. 45), but these become 

 distinguishable in the prophase of the second maturation which is 

 rapidly passed over (fig. 46). 



The spindle of the second maturation division is quickly formed 

 and the chromosomes become arranged upon it as dyads (fig. 50). 

 The second maturation spindle in both methods of fixation can be 

 distinguished from the first by its smaller size and by the smaller size 

 of the chromosomes. Division, like that of the first maturation, is 

 equal, and the chromosomes may be seen in anaphase as separate, 

 spherical bodies (fig. 51), while in telophase they are a densely stain- 

 ing mass about which a nuclear membrane is forming (figs. 53 and 54). 

 The second division of the chromosomes is longitudinal and may be 

 traced back to the longitudinal split of the spireme. The centrosomes 

 and their accompanying structures show much the same behavior 

 in the second as in the first division ; the cells increase greatly in length 

 and a constriction, which is marked by the zwischenkorper, appears 

 at the equator. 



Since there is no unequal division of chromatin material in either 

 the first or second maturation divisions, it is clear that there is no 

 "accessory chromosome" and hence no visible dimorphism of the 

 spermatozoa; there are, however, reasons for believing that the 

 peculiarity of behavior of one of the bivalents during the growth stage 

 classes it with Nezara as an equal pair of idiochrom xsomes. 



5. The Idiochromosome. — While the other chromosomes have been 

 undergoing these changes of form, the idiochromosome described for 

 the growth stage as a single or double body (figs. 23-29), which reacted 



