440 



TREADIVELL. [Vol. XVII. 



Cell Mechanics . 



In view of the number of observations which have been 

 recorded by Mead (No. 22), Conklin (No. 5, a), Jennings (No. 16), 

 Wheeler (No. 32), and others, showing that none of the laws 

 of cell division, as formulated especially by Hertwig (No. 12), 

 are of universal or even of general application, further addi- 

 tions to the list may seem superfluous, and I will mention 

 but one case which shows that the karyokinetic spindle does 

 not invariably lie with its long axis in the direction of greatest 

 protoplasmic mass, or of least resistance. When the entoderm 

 cells begin to invaginate, they become elongated, and their 

 lower ends are very much compressed by the pressure of 

 surrounding cells. (See Text-Fig. 5.) During this process 

 the cells divide. Now it is evident that if division took place 

 at right angles to the greatest elongation of the cell, or in the 

 direction of greatest pressure, a two-layered entoderm would 

 result. A two-layered entoderm would be of no especial use 

 to the animal ; what is needed is a larger area of absorptive 

 surface, and that can only be obtained by a division in the 

 opposite direction ; in other words, in a direction such that 

 the spindle lies in the line of greatest pressure, and at right 

 angles to the greatest elongation of protoplasm. I have re- 

 peatedly seen spindles lying in these cells during the invagina- 

 tion process, and invariably they occupy the direction shown 

 in Text-Fig. 5. Many other examples of this nature might be 

 found in the cleavage of Podarke, emphasizing the fact that, as 

 Conklin pointed out, protoplasm is " not soapsuds or oil emul- 

 sion," and that dominating the developing embryo is some 

 force vastly more powerful in its influence on the embryo than 

 any merely mechanical forces, and providing that every spindle 

 shall take the direction necessary in order that a bilaterally 

 symmetrical organism may result. If there is no especial 

 reason why it should be otherwise, the spindle may lie in the 

 long axis of the cell, or successive spindles may alternate in 

 direction ; but if in order that a perfect organism shall develop 

 the spindles need to take some other position, that position 

 will be assumed. Not only is this force manifest in the direction 



