14 ORGANIZATION AND CELL-LINEAGE OF ASCID1AN EGG. 



The question as to the cause of the dissolution of the nuclear membrane is an 

 interesting one. In a recent work, R. Hertwig (1904) suggests that it is due to 

 the fact that the cytoplasm attacks the nucleus after the cell has ceased to grow. 

 From such observations as I have made 1 should be inclined to think that the 

 cause was a quite different one, viz., the continued growth of the nucleus at a 

 iin ire rapid rate than the cytoplasm. ' In most if not all cases the nuclear mem- 

 brane dissolves only alter the nucleus has exceeded in volume a certain ratio to 

 the cell bodv. In the ascidian egg the germinal vesicle does not begin to dissolve 

 as soon as the egg ceases to grow ; on the other hand, therv is a considerable period 

 after the maximum size has been reached before the nuclear membrane disappears; 

 during this period the germinal vesicle continues to enlarge, the test cells are 

 extruded, the secretion which gives rise to the chorion is poured out, the entire 

 egg shrinks in volume, and finally the nuclear membrane grows very thin and dis- 

 appears. This process is in no wise complicated by the presence of a centrosome, 

 since, according to my observations, no centrosomes are present at any stage of the 

 maturation divisions. 



2. Chromosomes. 

 Even before the wall of the germinal vesicle dissolves the chromosomes may 

 be distinguished as small deeply staining bodies, some of which at least are V- 

 or Y-shaped (fig. 70). They are small and numerous, and I have not been 

 able to count them with any assurance of accuracy. After they have been drawn 

 together into the center of the nuclear area, as described above, they become a little 

 larger and are plainly V-shaped (tigs. 62, 63, 77). When the spindle fibres appear 

 they are at first widely scattered on or among these fibres (figs. 63, 79, SO), and 

 only in the metaphase do they become arranged in an equatorial plate (fig. 66). 

 In the splitting of the chromosomes the daughter halves first separate at the apex 

 of the V and remain longest connected together by the two limbs ; this double V, 

 with the apices pointing to the two poles of the spindle, is then stretched out 

 until the two limbs of each V come to lie near together, thus forming a double Y, 

 each with a long stem pointing to opposite poles; even the little space between the 

 limbs of the Y may disappear, thus forming cross-shaped chromosomes (fig. 66). 

 After the daughter chromosomes have separated they are plainly V-shaped (figs. 67, 

 68) ; and this shape may also be seen in the first polar body and in the second 

 polar spindle (figs. 68, 69). In the second polar spindle each limb of the V is sepa- 

 rated from the other, thus giving rise to rod-shaped chromosomes, which are found 

 in all the stages of the anaphase and in the second polar body (figs. 70-73). 

 Owing to the small size of the chromosomes it has not been possible to deter- 

 mine with certainty which of these maturation divisions is reducing and which 

 equational. If the two limbs of the V's in the first maturation represent two indi- 

 vidual chromosomes united at one end, then the first maturation division is equa- 

 tional and the second reducing, for these limbs of the V's are not separated until 

 the second maturation ; if, on the other hand, the cleft in the original V's represents 

 the splitting of two original chromosomes placed side by side (a thing which seems 



