Agarella gracilis. 217 
It will be seen that the above description of the formation of the 
pansporoblast does not agree with that put forward for similar Myxosporidia 
by other writers, and a few remarks must be made to justify the inter- 
pretation put upon the figures. A lengthy discussion on the matter by 
me is unnecessary for two reasons, firstly, that should the figures not carry 
conviction then no amount of argument could do so, and secondly, that 
Auerbach [3] has so well discussed the various accounts of spore develop- 
ment in the Myxosporidia that any further general account would be 
superfluous. I will indicate briefly the points of difference between certain 
published explanations of the facts and the one which I have put forward 
above. 
As aresult of his observations, chiefly on Myaxidiwm bergense, Auerbach 
[3] comes to the conclusion that in the Myxosporidia the pansporoblast is 
formed by union of a micro- with a macro- gamete. In this zygote the 
nuclei either unite at once to form a synkaryon or divide separately to 
form the numerous nuclei of the complicated pansporoblast with either one 
(as in Myxidium bergense) or two (as in Myxobolus pfeifferi) spores. He 
carefully describes and discusses the various accounts by other authors of 
Myxosporidian spore development, and shows how they may be brought 
into line with his own observations. There are at least two serious 
difficulties, namely the accounts by Keysselitz [8] and Schréder [11 and 
12] of spore formation in Myxobolus pfeiffert and Sphaeromyxa sabrazest 
respectively. In each of these two cases union of two “gametoblasts” 
each with one large and one small nucleus (said to be formed by division 
of the one original nucleus in each gametoblast) is described as taking 
place to form the four-nucleate pansporoblast. But Mercier [10] has shown 
that Keysselitz’ binucleate gametoblast is formed by fusion of two distinct 
cells, micro- and macro- gamete, while Schréder in his original account 
described the binucleate gametoblast of Sphaeromyxa sabrazest as being 
formed in the same way. Both Mercier and Auerbach, therefore, came to 
the conclusion that the pansporoblast is formed by the union of an original 
micro- and macro- gamete only. Now Myzxidiwm differs from most of the 
Myxosporidia of which the spore development has been described in being 
a monosporous type, 7.e. in forming the spores separately instead of in pairs 
in pansporoblasts. We are concerned here with spore development in a 
disporous type, 7.¢. one in which two spores are developed in one pansporo- 
blast, and the steps of the process may be summarised thus :— 
1. Differentiation of nuclei in the plasmodium into two types, small 
and large. 
