244 E. W. MACBEIDE. 



from connection with the gut 5 the relative position of the adult 

 oesophagus {a.w.) is also -well shown. Fig. 60 is from a larva 

 of about the same age; it shows the formation of the fifth 

 perihsemal rudiment (ph. 5.1) as an outgrowth of the ventral 

 horn of the posterior coelom : this lies beyond the fifth 

 hydrocoele lobe, and will therefore come to lie between this 

 and No. 1 lobe when the two ends of the hydrocoele 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 {l"p"c") 

 of the left posterior ccelom wedges itself in between the gut and 

 the hinder wall of the anterior coelom (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 hydrocoele, the stone-canal. 

 The ciliated cylindrical epithelium of this has now become 

 continuous with that of the pore-caual, 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 

 ccelom 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 ccelom. 

 This groove we found became converted into a canal opening 

 into the hydrocoele on one side, and the anterior coelom 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 PI. XI, figs. 14 — 16. We notice that the 

 prseoral lobe or stalk has become very much reduced, and that 

 the two ends of both curves, that of the hydrocoele 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 



