No. 2.] THE SEA-URCHIN EGG. 457 



tion which results in the formation of both the chromosomes 

 and the spindle-fibres witJtin the nuclear membrane. The linin 

 network suddenly becomes very conspicuous, its substance 

 greatly increasing in amount and staining more deeply, while 

 the chromatin correspondingly decreases, becoming localized to 

 form a number of irregular rods or threads (Phototype 6, Text- 

 figs. IV, V). At first these bodies may be often traced directly 

 into continuity with the less deeply stained linin fibres. Later 

 they become sharply defined and finally break up to form 

 thirty-eight (thirty-six T) chromosomes, which have the form of 

 slightly curved or nearly straight rods which arrange them- 

 selves in the equatorial plane of the spindle (Text-fig. VI). 

 Pari passu with the formation of the chromosomes the spindle- 

 fibres are differentiated out of the linin network, as already 

 described. It is evident from the foregoing description that 

 there is no proper Knauel or spirem stage, and I can discover 

 no evidence of splitting until after the chromosomes have been 

 fully formed. 



The main point of interest is the sudden increase in the 

 substance of the linin network at the close of the pause. It is 

 possible that this appearance is due simply to a rapid conden- 

 sation and localization of the chromatic substance whereby the 

 linin network is brought more clearly into view. A careful 

 study of the nucleus at every step of its transformation leads 

 me, however, to regard this as improbable, and I find it diffi- 

 cult to escape the conclusion that a considerable portion of the 

 chromatic portion of the original chromatic substance breaks 

 down into linin, and is thus ultimately transformed into spindle- 

 fibres. This result is less anomalous than it may at first sight 

 appear ; for Heidenhain has shown ^ how intimately related 

 chromatin and linin probably are in a chemical sense, and 

 there are a number of well-determined cases in which a con- 

 siderable portion of the chromatic network degenerates and is 

 converted into " achromatic " nuclear material or into cyto- 

 plasm, without entering into the formation of the chromo- 

 somes (as for instance in the formation of the polar bodies, 



1 Neue Untersuchungen iiber die Centralkorper. Arch. Mik. Atiat., 43, pp. 

 542-549- 



