OBSERVATIONS ON THE MATURATION PROCESSES. 27 



there are granules scattered throughout its substance and the chromo- 

 somes are still distributed over its surface. Sometimes the surrounding 

 cytoplasm shows a faint radial structure, which has the axis of the spindle 

 at its center (fig. 7). 



Stage IVa (plate 2, figs. 8, 8a, 8b, and 9). In this stage the chromo- 

 somes are drawn into the region of the equatorial plane, some lying at 

 the surface and some nearer the axis of the spindle, where all make up 

 a cluster having the form of an uneven disk, the so-called equatorial 

 plate. The spindle fibers are still very delicate. Occasionally the radial 

 structure of the surrounding cytoplasm seen in the preceding stage can 

 still be observed (fig. 8). 



Stage IV b (plates 2 and 3, figs. 12 to 14). The chromosomes, still 

 near the plane of the equator of the spindle, are sometimes visibly attached 

 to the spindle fibers, which are now much more easily seen. However, the 

 chief characteristics of this stage are the tormation of several circum- 

 polar bodies at each end of the spindle and the appearance of a clearer 

 cytoplasmic region surrounding the spindle on all sides. The spindle in 

 this stage begins to elongate and to become correspondingly narrower. 



Stage V (plates 3 and 4, figs. 14 to 17). This stage is characterized 

 by the division and separation of the chromosomes (metaphase and ana- 

 phase of nuclear division). Fig. 14 shows several chromosomes already 

 divided into halves, while others are in process of separation. Figs. 15,16, 

 and 1 7 show more advanced stages in the migration of the daughter chro- 

 mosomes toward the poles of the spindle and also an increasing diminution 

 in the number and size of the circumpolar bodies and in the extent of the 

 clear region in the neighboring cytoplasm. The more advanced represent- 

 atives of this stage (figs. 16 and 17) show thickenings of the interzonal 

 filaments midway between their ends, and also the beginning of the con- 

 striction which cuts off the first polar cell. 



Stage VI. Telophase of First Spindle and the First Polar Cell. 



(Plate 4, fig. 18.) 



In this stage the daughter chromosomes, both in the egg and in 

 the polar cell, have fused into compact masses, which are still joined 

 to each other by the interzonal filaments. The middle thickenings of 

 the filaments have united to form the "cell plate," which is continuous 

 at its edge with the vitelline membrane where the latter has been con- 

 stricted to form the neck of the polar cell. The circumpolar bodies have 

 disappeared and the clear cytoplasmic region is very pale. 



2. Chromatin Parts of First Maturation Spindle. 

 The origin of the fundaments of the chromosomes has already been 

 described (p. 26). Although we are unable to state how these funda- 

 ments are formed from the chromatin of the germinative vesicle, the 

 changes by which they are converted into the characteristic mature 

 chromosomes can be traced with a fair degree of certainty. 



