16 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



Nemec, 1899). Finally, in the metaphase all the spindle fibres are drawn together 

 at the poles; but even in this stage, though the spindle fibres lie close together, 

 they do not unite into a central body, and there are no astral rays (fig. 66). In 

 the anaphase a few rays may be seen running from the margin of the chromatic 

 plate toward the equator of the cell and lying on the periphery of the spindle (figs. 

 67, 81, 82). These are the only fibres which are not continuous from pole to pole, 

 and are therefore the only ones which bear even a remote resemblance to astral rays ; 

 that they are not such, however, is shown by the fact that they radiate from no 

 center but lie only around the periphery of the spindle. In this respect they more 

 closely resemble peripheral spindle fibres than astral rays. Still, if one considers 

 that one of the characteristics of peripheral spindle fibres is that they are attached 

 to chromosomes, it will be seen that these fibres do not belong in that category. Of 

 course, since centrosomes are not present, there can be no central spindle. We have 

 in this case, therefore, a mitotic figure in which are neither central spindle, periph- 

 eral spindle nor astral rays in the strict significance of those terms. The spindle 

 which is present arises wholly from nuclear linin, and consists almost exclusively of 

 fibres which are continuous from pole to pole. 



The small size of the maturation spindles of the ascidian egg is notable as con- 

 trasted with the great size of the germinal vesicle. Among many annelids and mol- 

 lusks the first maturation spindle is at least as long as the diameter of the germinal 

 vesicle, whereas among the ascidians it is scarcely more than one-quarter as long. 

 However, in those animals in which the spindle is very long in the prophase or 

 metaphase it undergoes a great shortening in the anaphase, e. g., in Crepidula it 

 is not more than half as long in the anaphase as in the metaphase (Conklin, 1902). 

 This is probably true of all cases in which the maturation spindle is a large one ; 

 for, since division of the cell body regularly takes place through the equator of the 

 spindle, the spindle must be relatively short at the time of the division of the cell 

 body, or the polar body will be relatively large. In all those cases in which the 

 first polar spindle is a long one, centrosomes are present near the periphery of the 

 germinal vesicle before its membrane disappears and the loose linin network of the 

 nucleus is transformed into the spindle fibres, thus forming a large, loosely con- 

 structed spindle. Later, by contraction of these fibres, the spindle shrinks in all 

 dimensions. In the ascidians, on the other hand, no centrosomes are present and 

 the shrinkage of the linin takes place before the spindle is formed, so that from the 

 first it occupies but a small part of the volume of the germinal vesicle, and is no 

 larger at the beginning of mitosis than at its close. 



The second maturation spindle arises in part at least from the remains of the 

 first, and is about half as large. Here also there is no trace of centrosomes or astral 

 radiations at any stage. The spindle is barrel-shaped, and a few peripheral fibres 

 are found around it in the anaphase (figs. 69-72) ; in all respects it closely resem- 

 bles the first maturation spindle. 



Such a case of mitosis as this, in which we have the formation of a spindle, the 

 separation of chromosomes and the division of the cell body entirely without the 

 presence of centrosomes, offers a valuable opportunity for the study of the mechanics 



