No. 3.] THALASSEMA AND ZIRPHAEA. 599 



culty. With the commencing fusion of the nuclei, the centro- 

 somes take up a polar position, and immediately become the 

 centers of renewed activity, for many additional rays commence 

 to start up about then. 



From the above it is quite evident that the centrosomes persist 

 entire throughout the whole of the critical stage where, in so 

 viany forms, they have been lost sight of (cf. Figs. 32-37). 



d. Tlie Cleavage- Amphiaster. 



After taking up a nearly central position, with its axis perpen- 

 dicular to the egg-axis, the cleavage-nucleus rapidly elongates, 

 sometimes even before the pronuclei have completely fused. 

 Meanwhile a minute centrosphere becomes differentiated about 

 each centrosome (PI. XXXIII, Figs. 37-39), which has persisted 

 unchanged up to this period, with the rays converging directly 

 to it. The centrosomes remain single until shortly before 

 metaphase, when each divides (PI. XXXIII, Fig. 39); but the 

 daughter halves do not diverge to any extent. The inner rays 

 soon commence to push in the membrane at the poles, and 

 later throw it into folds (PI. XXXIII, Fig. 38) in the manner 

 described by Platner (Anlastomum), Watase {Squid), Braus 

 (Salamander), and others. These in-growing spindle-fibers are 

 essentially similar in behavior to those of the first maturation- 

 asters. Like the latter, they are first similar to the remaining 

 astral rays, but among them traction-fibers are soon distinguish- 

 able by their greater thickness and homogeneity (PI. XXXIII, 

 F'gs. 39-41). For some time after the nucleus has become 

 fusiform, with the chromatin-spireme crowded together equato- 

 rially, the membrane still persists laterally and conforms strictly 

 to the astral system. Later its two halves merge into and 

 become indistinguishable from its rays. 



Equatorial sections during metaphase (PI. XXXIII, Fig. 40) 

 show the chromosomes arranged in a solid plate set in the center 

 of a circular area of granules (cross-sections of spindle-fibers 

 and those astral rays that pierce the equatorial plane), in all 

 respects similar to the first maturation-spindle, save the double 

 number (24) of chromosomes. Immediately upon divergence 



