No. 3.] THALASSEMA AND ZIRPHAEA. 62 1 



manner, and may be further complicated by becoming curled, 

 coiled, twisted, or bent in a zigzag manner. Their rough and 

 granular composition is quite apparent. Analogy with other 

 forms possessed of ring chromosomes leaves little doubt that 

 these are the product of several transverse and one partial 

 longitudinal division of a spireme thread. 



By the concentration of the chromatin, observed so fre- 

 quently at this period in other forms (e.g., Copepods, Hacker, 

 '95 ; Insects, vom Rath, 92 ; Wilcox, '95 ; OphryotrocJia, Kor- 

 schelt, '95 ; Selachians, Riickert, '93, etc.), each chromosome 

 entirely loses its granular appearance and becomes converted 

 into a compact tetrad, which consists of four closely apposed 

 loops or spheres (PI. XXXIV, Figs. 49 and 50). 



Along with the rings and rods in the germinal vesicle, cross- 

 shaped chromosomes are also to be met with, and the arms of 

 these may be straight or more or less bent or curved so as to 

 recall the " ophiurid-shaped " chromosomes of Hertwig ('90) (PL 

 XXXIV, Figs. 47 and 49). The arms seem never to exceed 

 four in number. In favorable preparations these crosses are 

 seen to be hollow in the center, with indications of a split or 

 seam extending up into two or more of the arms. These chromo- 

 somes may in consequence be regarded as rings compressed 

 along two mutually perpendicular diameters, with the four 

 included quadrants thereby converted into loops. Additional 

 evidence is here furnished of the correctness of the interpreta- 

 tion given to the crosses in Thalasscvia (p. 607). 



From the open rings it would seem that the tetrad may 

 arise directly by concentration of material at four different 

 points on the circumference. While this may possibly take 

 place sometimes by a looping, such pictures as PI. XXXIV, 

 Fig. 47, seem to show that the tetrad arises by a curling or 

 tangling up of the ring into a knot at the four points. The 

 result, however, is essentially the same in all cases — a further 

 concentration or contracting of the loops gives rise to a fairly 

 compact tetrad. Upon reaching this stage, or sometimes while 

 still "ophurids," the tetrads commence to enter the forming 

 spindle, and often become thereby temporarily elongated again 

 or variously distorted (PI. XXXIV, Fig. 49). 



