302 E. W. MACBRIDE. 
from connection with the gut; the relative position of the adult 
cesophagus (a.@.) is also well shown. Fig. 60 is from a larva 
of about the same age; it shows the formation of the fifth 
periheemal rudiment (ph. 5.1) as an outgrowth of the ventral 
horn of the posterior celom: this lies beyond the fifth 
hydroceele lobe, and will therefore come to lie between this 
and No. 1 lobe when the two ends of the hydroceele meet. We 
also see the process of destruction of the stalk going on, the 
ectoderm of its anterior surface being invaginated in patches, 
and, as we shall see, each patch as it is invaginated becomes 
destroyed by histolysis. Fig. 61 is from a larva which has 
nearly attained Stage F; it shows how the dorsal horn (/’p’c’’) 
of the left posterior coelom wedges itself in between the gut and 
the hinder wall of the anterior celom (a’). In this wall we see 
running from left to right (i.e. from oral to aboral sides of the 
disc) from the second lobe of the hydroceele, the stone-canal. 
The ciliated cylindrical epithelium of this has now become 
continuous with that of the pore-canal, but only on one side ; 
the conjoined tubes still open to the anterior coelom, and this 
opening persists in the adult, a fact which Ludwig did not 
observe (to see this, a more dorsal section than fig. 61 would 
have to be shown). The reader will remember that the pore- 
canal is formed by a dorsally directed outgrowth of the anterior 
celom fusing with the ectoderm, and a perforation occurring at 
the point of contact, and that the stone-canal is at first a ciliated 
groove running along the posterior wall of the anterior ceelom. 
This groove we found became converted into a canal opening 
into the hydroccele on one side, and the anterior celom on the 
other just below the inner opening of the pore-canal (woodcut 2). 
We have now arrived at Stage F, the external appearance of 
which is shown in Pl. 18, figs. 14—16. We notice that the 
preoral lobe or stalk has become very much reduced, and that 
the two ends of both curves, that of the hydroceele lobes 
(numbered in Arabic figures) and that of the arm rudiments 
(numbered in Roman numerals), have become very much ap- 
proximated to each other. 
At the same time we see that oral and aboral parts of the 
