THE PROPHASES. ORIGIN OF THE KARYOKLYETIC FIGURE. 23 



amount at this time. From this network the siaindle-fibres are formed ; hence the evidence indicates that the 

 spindle-fibres are indirectly derived from a portion of the chromatin which is first converted by a chemical change 

 into linin and then, by a morphological rearrangement of the latter, transformed into spindle-fibres. The remaining 

 portion of the chromatin retains its staining power and is finally transformed into the chromosomes, as follows. 

 Most of the residual chromatic network is converted into irregular rods and strings composed of lineally arranged 

 chromatin-granules (Text-fig. XIII. B, C). A part, however, gives rise to hollow spheres or rings of various sizes, 

 which lie scattered about among the chromatin-rods (XIII. A, B, C). 



It is very difficult to determine the precise relation of the chromosomes to these two kinds of bodies. Some 

 of the chromosomes appear to arise by the breaking of the rings at one side to form U-shaped rods which 

 finally open out as nearly straight chromosomes ; at any rate, every intermediate stage occurs between the closed 

 rings and U-shaped, curved, and straight rods. It is probable, however, that many of the chromosomes (possibly all 

 of them) arise from the longer rods, either by transverse division or by simple shortening. 



When fully formed, the chromosomes have the form of rather short rods, at first more or less curved, which 

 are grouped in the equatorial plane of the spindle (Text-fig. XV.). These chromosomes are 38 (possibly 36) in 

 number. It is not possible in this case to distinguish between paternal and maternal chromosomes, since their 

 individuality is entirely lost to view during the pause, owing to the complete fusion of the germ-nuclei. There can 

 be no doubt, however, that one-half the chromosomes of the equatorial plate are derived from each sex as in 

 Ascaris or Ptcrotrachca (cf. p. 5). 



, Plate VI. Phototype 24. 

 Origin of the Spindle. Differe^Uiation of the Rcticnltan. 



The specimen clearly shows the spindle-fibres passing from the aster at each pole into the interior of the 

 nucleus. The microsomal structure of the astral rays is clearly apparent. The small vague dark mass shown in 

 one of the asters is a group of granules (centrioles) like those shown in Text-fig. XIII. A, which form an early 

 stage in the development of the reticular centrosphere (cf. foot-note, p. 20). 



The phototype shows, further, the differentiation of the nuclear reticulum into a more deeply staining part, 

 the chromatin, arranged in strings and rounded masses, and a less deeply staining portion, the ///////, into which the 

 spindle-fibres are continued (cf. Text-fig. XIII. B). 



Plate VII. Phototype 25. 



Early Stage of the Karyokinetic Figure (jj minutes). 



The nuclear membrane is here disappearing, but still remains at the upper side. The spindle, now clearly apparent, 

 is separated from the remains of the nuclear membrane by a distinct interval. 



The chromatin appears both in the form of rods and rings (cf. Text-fig. XIII. C). One of the latter is clearly shown. 



