MUSEUM OF COMPAEATIVE ZOOLOGY. 269 



regards as protoplasmic. He considers the cell nucleus as nothing more 

 than an encapsuled portion of the active cell protoplasm, which may on occa- 

 sion become free by a rupturing of the capsule into two or more pieces. He 

 has observed, for instance, that in certain colorless blood corpuscles of Triton 

 and the frog the nuclear envelope becomes broken through, and the intranuclear 

 network and the cell protoplasm are thereby apparently in direct continuation. 

 " The ruptured caps;ile rests on an amoiboid mass (nuclear contents), like a 

 snail shell on a crawling snail." In the case of naked, for the most part un- 

 changeable, nuclei from the blood of the frog, Strieker adopts the term " Kern- 

 substanz " for the envelope and network of the nucleus (since they appear to be 

 alike), and " Kernsaft " for the clearer mass which fills the interstices of the 

 network. These naked nuclei are, however, genetically connected with uni- 

 nuclear cells, inasmuch as the latter are seen to change in a manner which 

 permits no other assumption than that the protoplasmic zone has withdrawn 

 within the nuclear envelope. The retiring protrusions become smaller, and 

 finally the nutjleus appears naked. The protoplasmic filaments often break 

 forth afresh, and the whole is again in motion. On such naked nuclei repeated 

 attempts at division were observed. From all this it is to be concluded that 

 the free nucleus with active (beweglich) internal network is only " ein abge- 

 kapselter ZelUeib," and that the capsule is perforated or permeable. 



In the blood of Triton and the frog there are still other elements, — finely 

 granular, " sehr beweglichen," colorless blood corpuscles. In these the nuclei 

 are not constant structures ; they appear and disappear, and again are formed 

 in the cell out of components of the cell body. The nuclear membrane is 

 only a transitory formation, like the waves on water. While one portion of 

 the nuclear membrane becomes invisible, a neighboring zone of the protoplasm 

 is compacted (new nuclear membrane) ; the nucleus has thus become larger or 

 smaller according to the position of this zone. The nuclei of these blood cor- 

 puscles, then, are not persistent formal elements. 



The nuclei of the tissue cells, or fixed cells, are less changeable, but even 

 here the network is sometimes (ciliate cells from the frog's palate) amoeboid, and 

 the nuclear membrane may change form, though it is not known to disappear. 



" Als Merkmal der fixen Zelle mag der Kern noch von Bedeutung sein ; als 

 ein nothwendiges Merkmal der beweglichen Zelle kann ich die Existenz eines 

 formell abgegrenzten Kerns nicht mehr anerkennen." 



The optical effect of the reticulum in the living nucleus is quite other than 

 that in the dead nucleus. In the latter one may speak of a " nuclear sub- 

 stance " and a " nuclear sap." In the living nucleus there must be, just as in 

 the cell, an intranuclear fluid in the form of very small vacuoles ; but it is not 

 to be considered that the fibres of the net-work are bathed in the living cell by 

 a " nuclear sap." The effect, on the contrary, is the same as though the reticu- 

 lum were produced by a special arrangement of the living material, — as it 

 were by an unequally distributed density of that substance. 



Strieker considers as most important his observation of the disappearance and 

 reappearance of the nucleus (active blood-corpuscles), and finds particular assur- 



