34 CELL-DIVISION AND GROWTH II. i 



opposite end. The line of union of the two pronuclei therefore 

 lies in the long axis of the egg". Nevertheless the fertilization 

 spindle is not formed in the minor axis of the ellipsoid as one 

 might expect. The two pronuclei rotate together through 90, 

 the spindle is developed, as usual, in a direction at right angles 

 to their line of union, that is to say the axis of the spindle lies 

 in the major axis of the egg, and the rule is confirmed. There 

 is a similar rotation of the fertilization spindle in the egg of 

 another Nematode, Diplogasier (Ziegler), and in the Rotifer 

 Atplanekna the spindle, at first oblique, becomes later coincident 

 with the long axis of the ovum (Jennings). 



Curiously enough, this rotation of the pronuclei does not occur 

 in another ellipsoid egg, that of the Rotifer Callidina. According 

 to Zelinka, the polar body is formed at one end of the long axis, 

 but the fertilization spindle lies in the minor axis, the first 

 division includes the major axis, and the law is disobeyed. After 

 the division, however, the cells rotate, and the plane of contact is 

 then, as in Ascaris nigrovenosa, transverse. 



Again, all polar divisions violate the rule, as also does the first 

 division of the fertilized egg of Ascaris megalocephala, and the 

 division of the cells of the germ-bands of Crustacea parallel to 

 their length (Bergh). 



On the other hand, Hertwig has brought forward experimental 

 evidence in support of his generalization. In the eggs of the 

 Frog the directions of some of the divisions were altered by com- 

 pression between glass plates. The eggs were just allowed to 

 assume their normal position with the axis vertical. They were 

 then placed between glass plates and compressed. 



In the first series of experiments the plates were horizontal. In 

 such eggs the first furrow was meridional and vertical, the second 

 meridional and vertical and at right angles to the first. So far, 

 therefore, division was as in the normal egg. In the third division 

 the furrows were, however, not latitudinal and horizontal, but 

 nearly vertical, being parallel to the first furrow above, to the second 

 furrow below. The surface of contact, therefore, formed by the 

 furrows of this phase must pass through a meridional position in 

 the interior of the egg. The fourth furrows are latitudinal. Born 

 has repeated the experiment and confirmed this result (Fig. 19). 

 He adds, however, that the furrows of the third division pass 



