2/6 CELL-DIVISION AND DEVELOPMENT 



matter ; for it will not account for the eccentric position of the 

 spindle in the formation of the polar bodies or in teloblasts. Neither 

 will it explain the eccentric position of the horizontal spindle in such 

 cases as the first cleavage of the annelid fg^. In Nereis, for exam- 

 ple (Figs. 43, 122), the inequality of the first cleavage is predeter- 

 mined long before actual division both by an eccentric position of 

 the spindle and an inequality in the asters, neither of which can be 

 referred to an unequal horizontal distribution of the yolk.^ In this 

 and many similar cases we must assume more subtle causes lying 

 in the organization of the cytoplasmic mass, or rather of the egg 

 as a whole ; but these deeper causes still lie beyond our grasp. 

 Unequal division, which plays so important a part in development, 

 still therefore awaits a final explanation, and until this is forthcom- 

 ing we have but a vague comprehension of the primary factors of 

 growth. 



Hertivigs Development of Saeliss Laiv. — We have now to consider 

 two additional laws of cell-division formulated by Oscar Hertwig in 

 1884, which bear directly on the facts just outlined and which lie 

 behind Sachs's principle of the rectangular intersection of successive 

 division-planes. These are : — 



1. The nucleus tends to take up a position at the eentre of its sphere 

 of iujluenee, i.e. of the pj-otoplasniic mass in zoIiieJi it lies. 



2. The axis of the mitotic figures typically lies in the longest axis 

 of the protoplasmic mass, and division tJierefore tends to cut tliis axis 

 at a right angle. 



The second law explains not only the mode of division in flattened 

 eggs, but also the normal succession of the division-planes according 

 to Sachs's second law. The first division of a homogeneous spherical 

 egg, for example, is followed by a second division at right angles to it, 

 since each hemisphere is twice as long in the plane of division as in 

 any plane vertical to it. The mitotic figure of the second division 

 lies therefore parallel to the first plane, which forms the base of the 

 hemisphere, and the ensuing division is vertical to it. The same 

 applies to the third division, since each quadrant is as long as the 

 entire &gg while at most only half its diameter. Division is there- 

 fore transverse to the long axis and vertical to the first two planes. 



Hertwig's second law has caused much discussion and has been 

 shown to have many exceptions, as for instance in the cambium-cells 

 of plants and in columnar epithelium. While undoubtedly one of 

 the most important laws of cell-division thus far determined, it only 

 pushes the analysis a stage further back, and leaves unexplained 



1 In an earlier paper 1 made the erroneous statement that the first cleavage-spindle of 

 Nereis lies centrally in the egg. Later and more careful studies by means of sections prove 

 that this was incorrect. 



