No. I.] POLYCHOERUS CAUDATUS. 97 



The manner in which the chromosomes were formed in the 

 first maturation spindle seems to be exactly the same as here 

 described, but the parts involved are so much smaller that it 

 is more difficult to make them out ; also, since the duration of 

 time from the entrance of the sperm to the complete formation 

 of the first maturation spindle is very short, the number of sec- 

 tions obtained through the polar spindles are but few. In no 

 case did I obtain a section directly in the plane of the equatorial 

 plate during its formation, but several diagonal sections show 

 clearly that a structure similar to that here described is formed 

 in the maturation spindle. The true significance of this reduc- 

 tion of the quantity of the chromatin is diificult to explain. 

 Furthermore, the reduction occurs twice during the formation of 

 each spindle. First the clumps of chromatin are selected from 

 the nuclei and then but a portion of these clumps are taken to 

 form the chromosomes. There are very many cases recorded 

 in which but a small portion of the nucleus is utilized, and the 

 rest dissolved into the cytoplasm, but in this ovum the quantity 

 seems unusually great. 



I have endeavored to form some estimate of the relative 

 quantity in the chromosomes as compared with the bulk of the 

 nucleus, by taking a small apple about the diameter of the mag- 

 nified nucleus as shown in Fig. 7, and dividing it in halves, 

 quarters, eighths, sixteenths, etc. The seventh division, which 

 would be y^^ of the whole, would certainly afford many times 

 more material than is contained in all of the chromosomes in 

 the cleavage spindle, and a comparison of Figs. 22 and 34 

 shows that the material in the chromosomes of this spindle is 

 vastly more bulky than in the polar spindle. Since it is thus 

 clear that a very small fraction- — possibly not more than -e,\-^ — 

 of the chromatin substance of the germinal vesicle and later 

 of the pronuclei is preserved as chromosomes, while the rest 

 is dissolved, the conclusion is immediately forced on us either 

 that the chromatin of the chromosomes differs chemically 

 from the bulk of the chromatin or that the chromosomes are 

 protected mechanically from the dissolving properties in cyto- 

 plasm. When it is remembered that the chromatin of the 

 polar bodies (Fig. 27) remains intact for many cell generations 



6 



