KARYOKINESIS. 45 



IV. General Considerations and Comparisons. 



I propose to give in this section a brief synopsis of the changes which the 

 nucleus, centrosome and sphere undergo during the whole cycle of division in the 

 mollusks which I have studied ; to compare these observations with closely related 

 ones in other animals and to indicate the general conclusions to which these obser- 

 vations lead. 



1. The Nucleus during the Cycle of Division. — The history of the nuclear 

 changes during the cycle of division may be summarized as follows : (1) The 

 chromosomes, consisting of chromatin inclosed in a linin sheath, divide and move 

 to the poles of the spindle where they partially surround the spheres. (2) Here 

 they become vesicular, the interior of the vesicle becoming achromatic, though 

 frequently containing a nucleolus-like body, while the wall remains chromatic. 

 (3) These vesicles continue to enlarge and then unite into the "resting nucleus"; 

 the nuclear membrane is composed of the outermost walls of the vesicles, while 

 the inner walls stretch through the nucleus as chromatic partitions; the chro- 

 mosomal vesicles from the egg and sperm nuclei remain distinct longer than those 

 from the same nucleus. (4) The chromatin of these inner alveolar walls then 

 aggregates into threads, giving rise to a " chromatic reticulum," though the linin 

 still preserves, for a time at least, the alveolar structure. (5) The chromatin of 

 these threads then aggregates into spherules, which are connected together by linin 

 threads ; these spherules vary in size, and at first all are solid and stain alike. (6) 

 They then become hollow and are differentiated into oxy- and basi- chromatin. (7) 

 In the first maturation each of the basichromatin spherules, or bodies, grows into 

 an individual chromosome ; in the cleavage the basichromatin spherules unite into 

 several linear series, thus forming a segmented spireme. (8) The oxychromatin 

 spherules grow smaller and some are dissolved in the nuclear sap while others are 

 arranged in series on the linin threads into which they are transformed; these 

 threads with attached spherules form the spindle fibres. (9) During the differentia- 

 tion of the chromatin the nucleus swells in size and the membrane becomes less 

 chromatic, while the nuclear sap becomes more so ; the nuclear membrane then 

 dissolves at points opposite the centrosomes and linin, oxychromatin and nuclear 

 sap here escape. (10) The spindle, which at first fills the entire nuclear cavity, 

 then grows longer and slenderer and contains an interfilar substance; the nuclear 

 membrane entirely disappears ; the equatorial plate stage is then reached and the 

 cycle is complete. In a word, the daughter chromosomes absorb achromatic sub- 

 stances, and unite to form the nucleus, within which the chromosomes and spindle 

 of the next division arise, while nuclear sap and dissolved chromatin escape into 

 the aster and cell body. 



Taking up now in more detail some of the individual steps in this cycle : 



(a) Formation of Chromosomal Vesicles. — Growth of Daughter Nuclei. — 

 When the chromosomes have reached the ends of the spindle, and in some abnormal 

 cases even before this (see text fig. IX), they begin to absorb achromatic material 

 and to swell into spherical vesicles. Such vesicles are found generally, if not uni- 



