328 
E. W. MACBRIDE. 
button is exposed (16) d In those forms of mammals in which 
the embryonic area is invaginated into the vesicle, Rauber’s 
layer does not extend over the embryonic area, but only up 
to the edges of the invagination. These edges are thickened 
tnasses which cohere and form a great plug of ectoderm called 
the “Triiger.” 
The formation of the coelom in Amphioxus as five out- 
growths from the gut, viz. a median and two paired out- 
growths, is not without parallels amongst other Yertebrata, 
even in the present state of onr knowledge, and it is more 
than probable that if attention were once directed to this 
point, a method of development fundamentally similar to that 
of Amphioxus would be found to range throughout the 
whole of the Yertebrata. In the embryos of the shark 
Acanthi as there is found a bilobed outgrowth from the 
anterior end of the gut, which becomes separated off as a 
single pre-mandibular cavity, which divides into two cavities, 
from whose Avails the inferior oblique muscle of the eye and 
the superior inferior and internal rectus muscles are developed. 
In the mandibular arch lies the so-called mandibular cavity, 
the origin of Avhich has not been traced. From its upper 
extremity, Avhich extends forwards over the pre-mandibular 
cavity just as the collar-cavity of Amphioxus extends over 
the head-cavities, the superior oblique muscle is developed, 
Avhilst from its lower portion constrictor muscles of the throat 
are developed. A pre-mandibular cavity originating fi’om 
the gut has been described in the lizard, and by EdgeAvorth 
in the chick (14). Pre-mandibular and mandibular cavities 
have been described in Urodela, Elasmobranchii, and in all 
Arnniota in Avhich they have been looked for; in Anura and 
Dipnoi they are apparently represented by solid masses of 
cells, as Agar has pointed out (1). That the trunk-cavities 
* I had the pleasure of listening to Professor Hill's exposition of this 
2 >oint and of seeing photographs of his preparations at the Dublin 
meeting of the British Association in 1908. On referring to the ‘ Pro- 
ceedings ’ of the Association I regret to find that this interesting pa^Jer 
is reported by title only. 
