CELL-DIVISION AND DEVELOPMENT. 131 



It may be pointed out that these two laws are very different 

 fundamentally, since that adopted by Berthold is based on the 

 action of extrinsic causes, while Sachs' seems to depend rather 

 on the interaction of intrinsic forces. It seems probable, how- 

 ever, that Sachs' law is to be brought into correlation with the 

 principle which, according to Hertwig, 1 governs the direction of 

 the karyokinetic spindle. It is a well-known fact that the 

 division of the cytoplasm stands usually in intimate relation to 

 the karyokinetic phenomena, though not invariably, since kary- 

 okinesis may occur and result in nuclear division without an 

 accompanying division of the cytoplasm. The two phenomena 

 are, however, as a rule associated, and it is to be noted that 

 Hertwig has shown that in an ovum subjected to pressure the 

 spindle forms with its longer axis at right angles to the line of 

 pressure, i.e., in the direction of least resistance. The same 

 result has been obtained by Driesch 2 in his experimental stud- 

 ies on Echinoid eggs, polar pressure applied to these eggs con- 

 verting the third cleavage, which normally is equatorial, into 

 a meridional one, an oblong plate composed of eight cells lying 

 in the same plane being the result. It is interesting to note, 

 in passing, that a similar arrangement of the cells in the eight- 

 celled stage is normally found in the ova of Teleosts, and is due 

 to the pressure produced by the large yolk-mass, as is shown 

 by Morgan's 3 experiment of puncturing the egg-membrane and 

 allowing a certain amount of the yolk to escape, whereupon the 

 third cleavage became meridional as in normal Echinoid ova. 



It is possible, then, that both the laws already defined may 

 be due to the action of extrinsic forces, but it must be remem- 

 bered that these laws do not account for all the phenomena 

 shown in the formation of cleavage-planes, and that there are 

 numerous cases which cannot be brought into harmony with 

 them. Thus it is well known to every embryologist that cells, 

 even when destitute of yolk, do not by any means divide 



1 Hertwig : Welchen Einfluss iibt die Schwerkraft auf die Theilung der Zellen ? 

 Jena, 1884. 



2 H. Driesch : Entwicklungs-mechanische Studien, III-VI. Zettschr. fur 

 wissensck. Zool., LV, 1892. 



3 T. H. Morgan: Experimental Studies on Teleost Eggs. Anat. A>iz. y VIII, 

 1893. 



