PRESENT PHOIU.KMS IX EVOLUTION AND HKRHDITY. 357 



uiuU'i' these iu'clio-plasiuic lorccs. is Ix-autiriilly sliown in <';ini(i\"s din 

 gTains (Fig. 7), First, the iiueh'ar wall breaks np. then t!ie clnoinatiu 

 coil unfolds into lines of vertical striation whicdi beeomc t iiread-like, 

 lienee the term mitosis, and then more compact, until linally a number 

 of distinct vertical rods, chr<mnitin rods, or clironiasomes are found. 



A remarkable and sioniflcant fact may be noted here, that the num- 

 ber of cliromasomes varies in the cells of diffei-ent s])(H'ies, and even 

 in the cells of difl'erent varieties (as in the tliread-worm of the horse — 

 Ascaris mepahxrpJiala), but is constant in all the cells of the same va- 

 riety tlirou|ih all stajies; thus the same number of chrcmiasomes ap- 

 pear in the lirst segmentation of the fertilized ovum as in the subse- 

 quent cell division in the tissues. 



Carnoy next indicates the veitical sjtlitting of ea<-h i-od into a loop 

 or link preceding the horizontal splitting: thus we may conceive of a 

 thorough re-distribution of the chromatin before it i>asses into the 

 daughter-cells. The split loops are each letracted toward a centro- 

 some, suggesting to some authors a contractile i>o\ver in the arclio-j)la8- 

 mic tilameuts, each chronmsome being appariMitly withdi'awn by a 

 single filament. But as tin' chroma-S(mies separate, the filaments also 

 appear b«^t\veeu them, ami are variously t<'rmed 'interzonal," "verbin- 

 dungs fiiden." '' filaments rt'unissant;" there is ther<'fore sonu^ differ- 

 ence of opinion as to what the nuM'hanics of the chromasome di\isions 

 really are. The chiomatin is now retra(;ted into two coiled threads, 

 each the center of the daughter nucleus with a single centrosome lie- 

 side it. r>ut as the line of cleavage is drawn hetween the two cells 

 (Fig. !>), the single centi-osome in each cell divides so that each 

 daughter-cell is now complete with its chromatin coil and two archo- 

 plasmic centrosorm-s. This jnocess has been beaut ifull>' (lescrihcd by 

 Watase.* 



rt thus appears thai both I lie chromatiiL and archo-plasm are perma- 

 nent elements of the cell, such as we formerly considered the luu-leiis: 

 the ai)parently passi^c chiomatin is divided with great jtrecisiou by the 

 active archo-plasm, then the archo-idasm sim])ly splits in tw^o to resume 

 the clea\age function. 



FertUizatioii — fin- iniioii of hereditary HKhstaiices. — l>efore looking at 

 the host of (pu'st ions which tertilization suggests, let us revieAV a few 

 of the well kn(»wn phenomena ])reparatory to the union of the germ 

 cells in order to gi\(' gi'cater emphasis to the importance of rec<'nt dis- 

 coveries. 



First, the o\um is a single cell, the typical structnic of which, with 

 its nucleus and cytoplasm, is ;j;(Mierally obscured by a (pumtily (»f food- 

 material, surrounded by a rather dense cell wall. The ovum is said to 

 be ripened or "matured" for the reception of the spermatozoon, by the 

 extrusion of two small "iiolar bodies," containing lioth chromatin and 



*See Marine Biological Laboratory Lectures, 1889. Boston: Ginu &. Co. 



