TWO ABNORMAL PLUTEI OF ECHINUS. 
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cavity, known as the amnion, is the result of a stimulus 
acting on the ectoderm and emanating from the hydroccele, 
and that the whole ectoderm of the larva is so organised that 
any part of it can form an amniotic cavity if the appropriate 
stimulus reaches it. Only in this way can we account for the 
formation of an amniotic cavity on the right side. The larva 
was cut into transverse sections, after being drawn, as shown 
in fig. 1 ; unfortunately the method of preservation (strong* 
formalin followed by strong alcohol) yielded such unsatis- 
factory results that nothing was learnt beyond what could 
be made out from the whole amount. I have come to the 
regretful conclusion that for delicate larvae like Echino- 
plutei no other reagent is available save osmic acid 
followed by Muller’s fluid, which occasions brittleness, but 
gives thoroughly satisfactory results as regards histology. 
We now turn to the second larva described in this paper. 
This is a larva of Echinus esculentus, fifty-five days old, 
which was preserved in alcohol. After making the two 
drawings of it, represented in figs. 3 and 4, I was attempt- 
ing to detach it from the slide for the purpose of having it 
cut into sections when it was accidentally lost. While 
regretting the accident, I consider it quite certain, from my 
experience with the other larva, that the preservation was 
too poor for any additional information to have been gleaned 
from sections. Eig. 2 represents a posterior view of this 
larva when it was thirty-five days old. This drawing was 
made from the living specimen by Mr. de Morgan, and my 
best thanks are due to him for his permission to use this 
sketch in illustrating my paper. The larva is normal in 
every way except for the presence of two “ echinus rudi- 
ments,” seen as dark lobed masses, one situated at each side 
of the alimentary canal. The alimentary canal is also dark- 
coloured, owing to the fact that it is crammed with Nitschia. 
The “ echinus-rudiment ” lying on the left side of the larva 
( ech .), that is, on the right side of the figure, is larger than 
the other abnormal one (ech., 1 ) which is situated on the right 
side of the larva, and consequently on the left side of the 
