48 KARYOKINESIS. 



but contribute directly to the formation of the linin threads and spindle fibres, as 

 Wilson ('95) and Griffin ('99) have found to be the ease in Toxopneustes and in 

 Thalassema. This is especially the case in all small nuclei, whereas the larger 

 the nucleus the greater the quantity of oxychromatin which dissolves and passes 

 into the cytoplasm. 



The solution of the nuclear membrane goes on coincident!)' with the solution 

 of the oxychromatin, so that it seems probable that the causes of the two are the 

 same. Before its solution the membrane changes its staining qualities, becoming more 

 and more plasmatic in reaction. The membrane is in all cases first dissolved at 

 points opposite the centrosomes. In this process the membrane undergoes one of 

 two modifications : either (1) it is drawn out toward the centrosomes into a cone-like 

 figure, or (2) it is indented opposite the centrosomes. Both of these methods may 

 coexist in the same animal, though one or the other is usually predominant. Among 

 mollusks of all classes, the membrane is usually indented. The difference between 

 these two methods is not great, depending upon the time at which the membrane is 

 dissolved and upon the rate of outfiW of nuclear substance; if the membrane is 

 thin and dissolves earl}- a cone is formed ; if it dissolves slowly and only after a 

 considerable quantity of nuclear substance has escaped, it becomes indented. The 

 strength of the nuclear membrane in Crepidula is shown not only by the degree of 

 indentation which it suffers before it is completely dissolved at the poles, but also 

 by its long persistence in the equator of the nucleus (see figs. 84 and 88). Even 

 when the membrane persists for a long time and becomes deeply indented at the 

 poles it need not be supposed that pressure is brought to bear by the polar fibres or 

 by other means upon the membrane ; on the contrary, the indentation is chiefly or 

 entirely due to the escape of nuclear sap and the consequent collapse of the nuclear 

 wall. . 



The infolding or outfolding of the nuclear membrane at points opposite the 

 centrosomes is a very common phenomenon among all classes of animals. It would 

 be useless to attempt to summarize all the observations on this point, and I shall 

 refer to only two recent works which touch upon this subject : — 



Montgomery ('98) has observed a cone-shaped protrusion of the nuclear mem- 

 brane opposite each centrosome in Pentatoma. These cones contain a dark sub- 

 stance which he believes to be of nucleolar origin. 



Fischer ('99) interprets the openings at the poles of the nucleus as due to a 

 greater growth of the nucleus at these points. The fact, however, that it first occurs 

 opjaosite the centrosomes and in connection with the formation of the half spindles, 

 indicates that the opening in the nuclear membrane is due rather to the solvent 

 action of some substance which diffuses to or from the centrosomes. 



(c) Escape of Nuclear Substances ; Aster and Spindle Formation. — It may 

 be considered certain that the infolding (or outfolding) of the nuclear membrane at 

 points opposite the centrosomes is due to an outflow of nuclear substance at these 

 points. This is conclusively shown by the fact that the linin reticulum, with its 

 attached chromatin granules, here extends outside the nuclear Avail nearly to the 



