152 GILBERT C. BOURNE. 
time were believed to be non-nucleate, have since been shown 
to contain nuclei, or at any rate nuclear matter, that we are 
tolerably well justified in saying that the nucleus, or its 
equivalent, is an essential constituent of the cell. At all 
events we know that division of the nuclear substance, whe- 
ther mitotic or amitotic, is all-important as a prelude to and 
accompaniment of cell division. The experiments of Gruber 
and Verworn show that if Ameebe are artificially divided, the 
parts cut off will regenerate and lead an independent existence - 
if they contain nuclear matter, but if they do not, they soon 
perish. Fragmentation of the nucleus—by which is produced 
a so-called multinucleate condition, often of considerable 
duration—is a prelude to spore formation, i. e. to the 
division of the cell into many parts. Mitotic division is highly 
characteristic of division of the cell into two parts. It is very 
difficult to draw distinctions, but it is worth consideration 
whether the temporary multinucleate condition ending in 
multiple fission, which is common in protozoa, has not a 
different value to the permanently multinucleate condition of 
some plants and animals, which are generally called unicellular. 
In the one case (e. g. Podophrya, Thalassicolla, Actinospherium) 
division or fragmentation of the nucleus leads, sooner or later, 
to the separation of cells, each containing a fragment of the 
original nucleus. In the Celoblastz (Siphonez, e. g. Caulerpa) 
the repeated division of the nucleus is not followed by any 
cell division, but the organism is throughout life a mass of 
continuous undivided protoplasm. ‘The plant, as von Sachs 
says, is of considerable size, develops roots, even leaf-forming 
shoots, and in its protoplasm hundreds and thousands of cell 
nuclei are contained, which with advancing growth are multi- 
plied by division, and obtain a definite arrangement within the 
protoplasm. And, as in the case in cellular plants, the nuclei 
are specially aggregated at the growing points. The whole 
behaviour is just that of a multicellular plant, but there are no 
partition walls. 
It is stretching the point very far to call this a single cell. 
And, in fact, it is an inconsistency to do so, for where, by an 
