Cytology of a new variety of A chly a americana . 169 
explained by regarding it, as Strasburger has pointed out, as 
an atavistic phenomenon. Objections were, however, taken 
to Strasburger’s view that reducing-divisions do not occur in 
the gametangia. The first direct evidence adduced in support 
of such divisions taking place was furnished by me in the 
case of Saprolegnia mixta . Since that time Farmer and 
Williams (’96, ’98) and Strasburger (’97) himself have succeeded 
in demonstrating reducing-divisions (halving of the number of 
chromosomes) in a highly satisfactory manner in the game- 
tangia of Fucus ; and Williams has recently discovered them 
in Dictyota , making public his results at the meeting of the 
British Association at Bristol. We have good, if not excellent 
evidence that similar divisions take place in Achlya. 
Strasburger, however, in the ‘ Cytologische Studied (’97), 
if I apprehend his meaning correctly, suggests that these 
divisions in the gametangia of the Thallophytes, in part at 
least, may be best explained by regarding them as essential 
features in the differentiation of gametes ; in the production 
of elements, that is to say, which have no independent power 
of germination, but whose capacity for further growth is 
limited by the necessity for a fusion in pairs. Such a view 
has no inherent improbability and it may well be accepted 
as highly probable. There is no reason, however, why such 
divisions should not, sometimes, at any rate, be reducing- 
divisions, and subserve two purposes at one and the same time. 
Let us realize clearly the facts which need explanation in 
the present state of our knowledge regarding such divisions. 
In animals there are gamobia only ; we have practically no 
sporobia. Reducing-divisions, so far as they have been 
elucidated, occur in the gametangia in oogenesis and spermato- 
genesis. In plants we have gametophytes and sporophytes. 
In plants higher in the scale than the Thallophytes we have 
a sharply marked antithetic alternation of generations. 
Whatever the origin of the sporophyte, whether by the 
intercalation of a new generation, or through the dimorphism 
of the gametophyte, we may regard the alternation as 
typically antithetic. In all these plants reducing-divisions, 
