No. 2.] THE EGG OF BUFO LENTIGINOSUS. 315 



hardly covers the facts. It is perfectly apparent that the 

 Cicatricula is caused by a lack of pigment granules in a definite 

 region of the upper surface of the &%%, so that the light-colored 

 substance beneath becomes visible at the surface ; but the 

 Cicatricula is often sharply defined when the debris from the 

 germinal vesicle is nearly absorbed and the yolk granules are 

 again uniformly distributed over this region. If the yolk 

 granules can again invade this region, why cannot the pigment 

 granules .-' Jordan suggests that an unknown repellent force is 

 exerted by the substance which forms the polar spindle and 

 later the female pronucleus, for it is only after the pronucleus 

 has moved away from the periphery of the egg that the Cica- 

 tricula disappears and the pigment is again uniformly distrib- 

 uted over the surface of the upper hemisphere. 



Sections through an Q^g from the body cavity (PI. XXX, 

 Figs. 29-31) invariably show the first polar spindle lying close 

 against the periphery, directly under the Cicatricula, and usu- 

 ally at the base of the small depression in the surface of the 

 ^gg. The spindle axis is generally approximately radial, but 

 it is often very oblique even during the anaphase (PI. XXX, 

 Fig. 32). In a longitudinal section of the spindle the chromo- 

 somes are sometimes found to be arranged in a row at the 

 equator (Fig. 29) ; more often, however, this stage has not yet 

 been reached and the chromosomes are scattered in groups 

 along the spindle (Figs. 30, 31), each group seemingly com- 

 posed of four rounded chromatin granules lying close together 

 and thus showing a striking resemblance to the tetrad groups 

 found by Boveri ('87) in the first polar spindle of the &gg of 

 Ascaris. A transverse section of the spindle at the stage of 

 the equatorial plate, or a longitudinal section of the spindle 

 during the anaphase (Fig. 32), will show conclusively that there 

 are no tetrad groups in the first polar spindle of the ^.gg of 

 Bufo. The chromosomes in Figs. 30 and 31 are in reality 

 dumb-bell-shaped loops arranged in pairs, with the angles of 

 the loops turned towards the center of the spindle, the knob- 

 like ends of a pair of chromosomes giving the appearance of a 

 typical tetrad group when the spindle is cut longitudinally. 



There are but twelve groups of chromosomes to be found in 



