142 Lloyd Williams. — Studies in the Dirty otaceae. 
publication of the results hitherto obtained. The present paper deals with 
the sporophyte generation, and the first segmentation of the spore. The 
cytology of the gametophyte and of fertilization, the very remarkable 
phenomena associated with parthenogenesis, as well as the natural history 
of the plant, will be dealt with in subsequent papers. 
Mottier has described and figured karyokinesis in the vegetative cells 
of the thallus. Our results being in general agreement it is not proposed 
to deal further with them. As, however, he has not described the stalk- 
cell division or the earlier stages of the reducing division, a detailed account 
of them will be given here. 
The Stalk-cell Division. 
One of the elongated rectangular cells of the thallus begins to swell 
in all directions, but chiefly outwards, in the direction of least resistance. 
At the same time the nucleus increases greatly in size. The nucleolus 
appears granulate, there is a fine reticulum uniformly distributed through 
the nuclear cavity, and upon it are numerous granules. The axis of the 
nucleus is as yet parallel to that of the thallus. At each pole there is a 
curved rod-like centrosome, and radiations extend into, and become merged 
in, the cytoplasmic reticulum. The chloroplasts stain uniformly ; they are 
much paler than those of the vegetative cells, and at first they are uni- 
formly distributed. Soon, however, the basal portion immersed in the 
surface layer becomes greatly vacuolated. In the vertical walls separating 
the cell from the neighbouring ones there are large pits and the vacuoles 
lie opposite them. 
The lateral extension of the cell soon ceases, but it grows rapidly out- 
wards until it is several times the depth of the cortical cell. In cross 
section the projecting part is nearly spherical, but a longitudinal section 
shows an elongated, dome-shaped mass. The dense cytoplasm, containing 
the bulk of the chloroplasts and the nucleus, is aggregated in the swollen, 
free portion. The axis of the nucleus has now swung round until it is 
vertical to the thallus, and the distal pole has a very distinct curved cen- 
trosome and beautiful radiations ; the space occupied by the latter being 
quite free of chloroplasts (PI. IX, Fig. 1). A cap of cytoplasm covers 
the basal pole. From this a cytoplasmic cord extends downwards into 
the vacuole, where it sometimes branches. The centrosome here is often 
only distinguished with difficulty and the radiations are completely absent. 
Stages where the axis of the nucleus is oblique to the thallus are very 
difficult to find, the change of position probably taking place very quickly. 
The nucleolus is now large and stains deeply. Sections through it show 
that it is uniformly fibrillate, or minutely vesicular in texture. 
Before the spirem there is a stage where the nuclear reticulum shows 
