6 INTRODUCTION. 



segment or daughter chromosome forms one-half of the ring, or 

 each maybe in the form of a short, thick U (Fig. 2, F). Sixteen 

 chromosomes, the reduced number, are present in the first mitosis. 



While on the way to the poles the daughter chromosomes sometimes 

 fuse with one another to form large masses. 1 This is especially so in 

 the second mitosis. 



In the construction of the daughter nuclei, one or more larger masses 

 of chromatin are formed by the chromosomes ; a nucleolus appears 

 near the chromatin mass or masses, and a nuclear membrane , is laid 

 down (Fig. 2, G). The membrane is unquestionably formed through 

 the agency of the kinoplasmic fibers. The centrosomes increase in 

 size, and the polar radiations are more distinct than in the spindle 

 stage. The connecting fibers usually persist until the nuclear mem- 

 brane is present, but a little later they disappear entirely. The chro- 

 matin mass, gradually becoming less dense, soon disintegrates, and 

 each daughter nucleus passes into the resting condition (Fig. 2, G). 



From the preceding it will be seen that each daughter nucleus is 

 provided with one centrosome, but in the first mitosis the centrosomes 

 could not be made out until they were on opposite sides of the nucleus 

 and provided with radiations. The question naturally arises : Does 

 the centrosome divide to give rise to the two daughter centrosomes ? 



Swingle ('97), who has traced the persistence of the centrosome 

 through several successive generations of vegetative cells in Stypo- 

 caulon, one of the Phceophycece, found that a division of the centro- 

 some takes place, and Strasburger ('97) arrives at the same conclusion 

 as regards Fucus. This is the generally accepted view. 



We shall trace the early development of the spindle in the second 

 mitosis in the tetraspore mother-cell in order to see what evidence is 

 furnished by Dictyota toward the solution of this problem. 



During the reconstruction of the daughter nucleus (Fig. 3, H) 

 two rod-shaped centrosomes, each with its radiations, were observed 

 close together, and in such a position as to form a wide V, giving the 

 impression that a longitudinal division of the single centrosome had 

 taken place. The manner in which a cluster of radiations is attached 

 to each daughter centrosome seems to lend weight to this conclusion. 



The daughter centrosomes now separate, moving along the nuclear 

 membrane, but they do not, as in the first mitosis, traverse an angular 

 distance of 180 before the formation of the spindle begins (Fig. 3, 

 I, K). The development of the spindle is the same as in the first 

 mitosis, as Fig. 3, I, J, K, L, will clearly show. 



In other brown algae, so far as known (Swingle '97, Strasburger '97) , 



1 This massing of the chromosomes may not occur in all cases. 



