CXXil 
It is trne that in this very paper M. Mecznikow expressly states no 
such body exists in Corixa, in Aspidiotus or in Aphis. It is, however, 
difficult to believe that so fundamental a difference should exist in 
the embryology of animals belonging to the same class, and in fact I 
might appeal to. one of M. Mecznikow’s own figures (pl. xxvili. fig. 11) 
to prove that, in direct opposition to his statement, pole-cells do oceur 
in Aphis. In that figure the blastoderm is represented, no doubt 
correctly, as a single layer of cells, except at the hinder end, where 
there are three extra cells. These three extra cells appear to me to 
be the pole-cells, in confirmation of which I would only ask any one 
to compare this figure with pl. xxiv. fig. 11, which represents the 
corresponding stage in Cecidomyia. It is true that the pole-cells of 
Aphis are not so dark or so large as those of Cecidomyia, but 
these are not essential characteristics, and on the other hand the part 
played by the blastoderm at this spot offers so much similarity, as we 
shall see, with that of the pole-cells of Cecidomyia as to leave very 
little room for doubt. 
The dark colour and large size of the pole-cells in Cecidomyia are, 
however, favourable conditions which have enabled M. Mecznikow to 
throw light on their history and functions, concerning which we have 
hitherto been in complete ignorance. Guesses have indeed been 
hazarded, but, as is usual in Science, they have all been wrong. The 
pole-cell, which is at first single, divides into two, and these again 
subdivide into four similar but smaller cells. In the mean time the 
blastodermic cells-have formed themselves into a regular membrane 
enclosing the yolk on all sides, and for some little time the pole-cells 
lie completely outside this membrane. Soon, however, they re-enter, in 
what manner is not stated, but are still easily distinguishable by their 
dark colour. They remain for some time without further change, and 
even when the embryo has largely increased in size, and thus become 
opaque, they can at any time be brought into view by a slight 
pressure. 
When at length the segments of the body are indicated, the 
cesophagus and rectum formed, and the rudiments of the mandibles 
and two pairs of maxille are already evident, the four pole-cells 
separate themselves into two groups of two, without, however, having 
undergone any further change. In embryos somewhat farther ad- 
vanced, each pair of pole-cells is found to be enclosed in a special 
organ occupying a definite position in the embryo. These organs are 
of an oval form and are composed of small cells, which do not differ 
