THE DEVELOPMENT OE THE TEMNOCEPHALE.E. 423 
surface^ so that they present the appearance, to some extent, 
of the nuclei of a superficial epithelium. 
The ovum (PI. 23, fig. 2) is embedded in the yolk-cells, 
usually towards the middle, sometimes towards one end of 
the egg. It is a polyhedral cell, ‘08 mm. in diameter, with a 
finely granular protoplasm that stains much more deeply than 
the substance of the yolk-cells. In all the specimens I have 
had the opportunity of examining the nucleus has undergone 
modification, and is represented by a cluster of clear vesicles, 
each enclosing a rounded particle with the staining affinities 
of chromatin.* 
In two cases I found an egg containing two cells. The 
nuclei of both had undergone the modification just described. 
One of the cells was very much smaller than the other, and 
on that account it seems to be more probable that we have 
here to do with the first staj^e of segfinentation rather than 
with an egg in which two ova had become enclosed. 
V. I'Iahly Develoi’men'I'. 
'Idle process of segmentation results in the formation of a 
blastoderm of irregular shape, which comes to be drawn out 
in the direction of the long axis of the future worm — a direc- 
tion, as already explained, usually parallel with the long axis 
of the egg, but not invariably so. No germinal layers are 
recognisable : but from a very early stage (PI. 23, fig. 3) the 
blastoderm is found to consist of three sets of cells, which 
differ from one another in a very marked manner in their size 
and in the character of their nuclei. The cells of one set are 
0'03 mm. in diameter, have nuclei about 0‘015 mm. in dia- 
meter, each containing a large rounded nucleolus. Those of 
the second set are 0‘12o mm. in diameter, have smaller nuclei, 
0‘0075 mm. in diameter, usually without nucleoli, but with a 
rather close network. The cells of the third or smallest set 
‘ Somewhat similar appearances were observed by Zeller (17) in 
Polystomum. 
