ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 23 



). The Egg Nucleus and its Movements. 

 After the formation of the second polar bodv the chromosomes left in the egg 

 form chromosomal vesicles which then unite to form the egg nucleus (fig. 73). The 

 latter then moves away from the animal pole into the yolk, apparently in the direc- 

 tion of the axis of the second polar spindle (figs. 86, 87); it soon turns, however, 

 and moves toward the sperm nucleus and aster at the posterior pole. A't first a few 

 remnants of spindle fibres connect the egg nucleus with the animal pole (fig. 87), 

 but these are soon lost and thereafter this nucleus, without any surrounding area 

 of protoplasm or astral rays, is almost lost to view in the dense mass of yolk (fig. 

 89). Finally the egg nucleus emerges from this yolk into the clear protoplasm 

 surrounding the sperm nucleus, and the two nuclei meet at the equator of the egg 

 about half way between the posterior pole and the center. The relative positions 

 of the two germ nuclei when they first meet is invariably the same ; the egg 

 nucleus always lying on the central (anterior) and animal pole (ventral) side of the 

 sperm nucleus (figs. 89-93). 



6. Sperm Amphiaster and first Cleavage Spindle. 

 About the time that the sperm nucleus has moved to the edge of the yellow cap 

 (fig. 8) and some time before the union of the two germ nuclei, the sperm centro- 

 some divides as shown in figures 88, 89. 1 have not observed all the details 

 of this division, but it is evident that the centrosome here gives rise to a centro- 

 somal spindle or netrum (Boveri L901 ). at the poles of which the daughter centro- 

 somes lie. After the centrosome has divided, the sphere also divides (fig. 88), and 

 a well-marked central spindle is left connecting the two daughter centrosomes (fig. 

 89). When these daughter centrosomes have moved to the poles of the sperm 

 nucleus the central spindle is curved around that nucleus, and finally its fibres 

 become indistinct (fig. 90) and then disappear altogether (fig. 91 ). The sperm aster, 

 at the time of its division, invariably lies on the central side of the sperm nucleus, 

 and the axis of the amphiaster thus formed is at right angles to the copulation path 

 and to the plane of the first cleavage (figs. 88-91). Up to the time when the 

 two germ nuclei meet, the sperm centrosomes lie at the poles of the sperm nucleus 

 (fig. 91), whereas no trace of centrosomes are ever found in connection with the egg 

 nucleus, and after the latter has moved away from the- animal pole, on its path to 

 the sperm nucleus, no trace of radiations, spindle fibres or even of surrounding 

 cytoplasm can he found near the egg nucleus. The cleavage centrosomes may be 

 traced without a break back to the sperm amphiaster, to the sperm aster and 

 finally to the middle piece of the spermatozoon. There could not possibly be a 

 clearer ease of the origin of the cleavage centrosomes from the middle piece of 

 the spermatozoon than is presented by the ascidian egg. This conclusion agrees 

 with the work of all who have ever studied the fertilization in these eggs (Boveri 

 1890, Julin 1893, Hill 1895, Castle L896, Golski 1899, Crampton). In Ciona, 

 Castle observed an archoplasmic mass in connection with each of the germ nuclei, 

 though that found near the sperm nucleus was larger and more energetic than the 



