PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 



355 



Mau])as upon tlio luultiplication and conjuiiation of the Iiil'iisoria, liiv^- 

 in^" 118 a liost of ncwideas as to the cycl«^ of life, tlio nicaninji' of sex, 

 and the oii.u'inof the sexual relation. 



In all this n'search and in the future outlook there are two main 

 questions : 



1. What is the hereditary substanee ? What is tlu^ material basis of 

 heredity, Mhieh sju'eads from the fertilized ovums to every (;ell in the 

 body, conveyino- its aueestral eharaeteristics! Is there any substaiiei^ 

 eorresponding' to the hypothetieal idioplasm of Niigiilil' 



2. What are its regulating- and distributing- forees ? How is the he- 

 reditary substance divided ;nid distributed? How far is it aetive or 

 passive 1' 



I may say at the outset that the idioplasm of Kiigeli, a purely ideal 

 element of protoplasm which he conceived of as permeating all the tis- 

 sues of the body as the vehicle of heredity, has been apparently mate- 

 rialized in the chromatin or highly coloring materials in the center of 

 the nucleus. This rests upon the demonstration by Van lieneden and 

 others that chromatin is found not only in all active cells, but is a con- 

 si)ieu(ras element in both the ovum and spermatozoon during all the 

 phenomena attending conjugation. 



Fig. 7.— Typical ckli, division, siiowiMi tiik ijistkiihtio.v ok chkomatin.— (From Tarkcr .after 

 Carnoy.) A-C, armiiucnii-ul of the cliroinatiii in tliroaiU; 1)-E, formation of the chromatin rods 

 aud loops; F, splittinjr of the- loops; (J-II, retraction of tln^ chromatin into tln^ two dau<:htor cells. 



Secondly, that while the chromatin is apparently passive, it is played 

 n\)<n\ l>y forces resident in the tjlear surrounding protoplasm of the 

 nucleus, bnt chiefly by the extra nuclear archoplasm, which seems to 

 constitute the dynamic and meehanieal factor in each cell. This, un- 

 like the chromatin, only comes into view when there is unusual activ- 

 ity, as during cell-division, and is not eN'ident (with our i)resent histo- 

 logical technique, at least), when the cell is arrested by reagents in 

 any of the ordinary stages of metabolism. 



Tlw (li.sirihiitioii of Inridilni-ji .siihsfaiirt . — I ma\' first review some 

 of the well-known jihenomena attending the distribution of the chro- 

 matin substance to the tissues. 



