MUSEUM OF COMPARATIVE ZOOLOGY. 377 



each new cell exhibits a differentiation into a more compact portion 

 immediately surrounding the nucleus, and a peripheral, less compact 

 portion.* The nucleus diflferentiates a nuclear membrane and nucleoli 

 of varying size, and the cells produce a cellulose envelope before their 

 mutual contact. 



In his " General Results and Considerations," based on animal as well 

 as vegetable cells,, Strasburger discusses first the stellate phenomena. 

 In free cell-formation there are in operation forces which, acting from a 

 central mass, attract most of the molecules of the surrounding proto- 

 plasm, but repel a small part of them, namely, those which compose the 

 " Hautschicht." These are molecular forces, and the radial arrange- 

 ment of the protoplasm favors the view of a polarity of the protoplasmic 

 molecules. In the eggs of animals, at the first formation of the " Keim- 

 kern," it is surrounded with rays. It is by means of these rays that the 

 nucleus pushes itself away from the peripheral Hautschicht, until, having 

 reached the centre of the eg^, it remains in equilibrium with its rays, 

 reaching out to the Hautschicht on all sides. It may remain in an 

 eccentric position only when the sphere of its operation is too limited to 

 cause it to take the central position. 



It seems to me unfortunate for this explanation that the influence of 

 the nucleus, as expressed in the length of the rays, should in some cases 

 diminish as it nears the centre of the egg. How, too, will it be possible 

 to explain the migration of the male pronucleus when, as in Limax, 

 there are no protoplasmic rays to push it from the periphery 1 



The function of the nucleus, he says, is made most obvious in cell di- 

 vision. It becomes homogeneous, then an opposition between two points 

 of its surface is developed. These points mutually repel each other, and 

 a lengthening of the nucleus is the result. Certain components of the 

 " Kernsubstanz " are repelled from each of the two poles, and are col- 

 lected simultaneously to form the nuclear plate. In proportion as the 

 substance of the nuclear plate recedes from the poles, a striation is dis- 

 tinguishable behind it. These changes within the nucleus induce the 

 radial alterations in the surrounding granular protoplasm. The changes 

 in the form of the cell are accomplished by the influence of the nucleus, 

 but only when the rays have reached the surface of the protoplasm 

 (Ascidia). 



Although the " Kernf aden " do not seem to have in animal cells an 

 extensive development or well-marked function, in plant cells they 



* This recalls the condition of the eggs of certain echinoderms, quite recently de- 

 scribed by Ed. van Beneden and Selenka. 



