254 BERTRAM G. SMITH. 



of the different cells is maintained to the late blastula stage, so 

 that there is little likelihood of confusing cell-generations. Be- 

 tween micromeres and macromeres there is a fine gradation in 

 phases of the nuclear cycle, so that a close series may be readily 

 obtained. The individuality of the germ-nuclei has been traced 

 without a break to an advanced cleavage stage, and it would not 

 be difficult to illustrate this entire result with an adequate 

 series of figures, but such a procedure would require an enormous 

 amount of time and unduly extend the limits of this paper. 



Beginning with about the seventh cell generation certain 

 irregularities, real or apparent, occur with increasing frequency 

 to mar the mechanical precision of the events thus far described 

 in the history of the germ-nuclei. In the resting stage the two 

 nuclear vesicles of a single nucleus are often deeply lobed, some- 

 times in such a manner as to give the impression that the nucleus 

 is made up of many vesicles. In the late telophases the nucleus 

 is often really made up of several or many separate though closely 

 aggregated vesicles, but this is undoubtedly a part of the usual 

 process of mitosis, as a more searching examination of these late 

 phases throughout cleavage will readily show. 



The transformation of the daughter-nucleus into the vesicular 

 condition must take place with considerable rapidity, especially 

 in the early cleavage stages, for some phases of this process have 

 not been observed earlier than the fourth mitosis, and only 

 occasionally in the fourth and fifth mitoses. In more advanced 

 cleavage there is little difficulty in finding material representing 

 the entire history of the telophase. 



In the early anaphase, the long thread-like chromosomes are 

 V-shaped, with the limbs of the V almost parallel and very 

 straight; the apex is directed away from the equatorial plate. 

 In a slightly later phase, which we may call the late anaphase, the 

 chromosomes are still roughly V-shaped but finely undulating 

 except at their free ends. A little later (early telophase) each 

 chromosome becomes much convoluted, oftei^ coiled in a loose 

 irregular spiral; this change begins at the apex and proceeds 

 toward the free ends. The next step consists of the meta- 

 morphosis of each chromosome into a chromosomal vesicle, a 



