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K. M. SMITH AND H. G. NEWTH. 
ventro-lateral extensions of the collar pouches ” — having 
observed this relation to hold in the case of the right collar 
cavity, and inferring that it was true of the left side also. 
Lankester and Willey (4) have described a pseudocoelic 
origin of the metapleural spaces, and MacBride, in answer to 
criticism by Lankester and others, returned to this subject in 
his paper of 1900 (2). Some of the conclusions arrived at, 
as a result of this re-investigation, were as follows : 
“The ectoderm on the external side of these ridges ” 
(i. e. the atrial ridges) “becomes thickened, the cells 
composing the thickening become clear and glassy and 
eventually are hollowed out to form a f lymph canal/ 
My former statement as to the coelomic nature of this 
lymph canal is therefore incorrect. 
“The extensions of the collar cavities into the atrial 
ridges become first separated off as the metapleural 
coelom on each side ; later this coelomic space becomes 
converted into a solid mass of cells from which arise 
muscular fibres in the neighbourhood of the gill openings, 
and almost certainly, later, the sub-atrial muscle.” 
In the following year van Wijhe (5) affirms: “Nach den 
Verlialtnissen beim ausgebildeten Thiere halte ich die Angabe 
von MacBride aus dem Jahre 1898, nach welcher die Seiten- 
flossenhohlen in Continuitat mit den f collar pouches * 
entstehen wurden, fur richtiger als die spatere Behauptung 
des selbstandigen Auftretens der Seitencanale.” 
Finally, MacBride, in his latest contribution to this subject, 
reiterates his former assertion — made in his second paper — 
that the collar cavities form spaces in the atrial ridges, 
longitudinally co-extensive with the pharynx, and distinct 
from the splanchnoccel (3). 
The present investigation was begun at the suggestion of 
Prof. MacBride, and was carried out in his laboratory. We 
wish here to make grateful acknowledgment of our indebted- 
ness to him for the assistance he has given throughout the 
progress of the work, and for the generous permission to make 
use of his preparations for purposes of comparison. 
