67 o Lewis . — The Life History of Grijfithsici Bornetiana. 
spore. The gelatinized cell-wall, now very much swollen, appears to 
dissolve in the sea-water, and the four spores are set free, almost imme- 
diately becoming spherical. Like the carpospores, they are heavier than 
sea-water, and slowly sink if left undisturbed. 
The mature tetraspore resembles the carpospore in appearance. It is 
approximately spherical. In the centre the large nucleus is conspicuous, 
with its chromatin segregated into 12-20 small masses. Immediately 
around the nucleus is a zone of rather dense cytoplasm ; outside this the 
cytoplasm is coarsely vacuolar. In the peripheral cytoplasm is a single 
layer of chromatophores, outside which is the limiting membrane of the 
spore. No cellulose cell-wall is visible. 
The average size of the tetrasporic structures is shown in the following 
table : — 
Cell. 
Nucleus. 
Mature mother cell 
15- 20 A* 
7 A* 
Mother cell at synapsis 
20X 2 \[X 
Six 
Four nuclei peripheral 
20 x 24^ 
4—6^ 
Four nuclei central 
25 - 3 ° A* 
4-6 fX 
Partitions separate 
40-50 A* 
8 A* 
The tetrasporic structures in a single sorus are of very various ages. 
While the first-formed tetraspores are developing, new tetraspore-mother- 
cells are being formed nearer the cross-walls between the vegetative cells, 
the older tetraspores being carried away from the cross partition by the 
growth and the stretching of the wall of the vegetative cell. A longitudinal 
section of a sorus shows primary tetrasporic cells being formed very near 
the point of junction of the vegetative cells; outside these are the older 
tetraspore-mother-cells ; farther out mature spores are to be seen ; while 
farthest from the centre occur the involucral rays (Fig. 133). 
The number of tetraspores produced in a single sorus is quite large. 
The average number in well developed sori was found to be about 300. 
The process of nuclear division in the tetraspore-mother-cell of 
Griffithsia offers many striking points of difference from the same stages in 
the life-history of Polysiphonia ; it resembles much more nearly similar 
stages in Corallina (Davis, 23 ). As these three forms are the only members 
of the Rhodophyceae in which the behaviour of the tetraspore-mother-cells 
has been carefully studied from a cytological standpoint, it may be well to 
summarize here some of the points of resemblance and difference : — 
