570 ADAM SEDGWIOK. 
while the anus is formed within the area of the blastopore, and 
is in some Vertebrates actually a persistent part of the blasto- 
pore, in no Vertebrate has the mouth been traced into connec- 
tion with the blastopore. The fact that no such connection 
has been established is not surprising when one remembers 
how early the anterior part of the blastopore closes in Hlasmo- 
branchs and Amphibia, and must not be taken as proving that 
the blastopore never extended in front of the present medullary 
plate on to the ventral surface of the head. I shall return to 
this question in the part of this paper which deals with the 
Vertebrate head. 
It will be seen from the above account that the behaviour of 
the blastopore of Elasmobranchs—in its relation to the anus, 
neurenteric canal, and growing point—resembles very closely 
that of the frog as described in the admirable paper by 
Assheton and Robinson in vol. xxxii of the ‘ Quarterly Journal 
of Microscopical Science.’ 
2. On the Formation of the Mouth and Gill-clefts 
in Elasmobranchs. 
I have had a number of drawings made of the head of 
embryos of Scyllium canicula to illustrate certain points 
in the formation of the mouth and clefts. Some of the points 
have been known before, and some are, I believe, recorded for 
the first time. _ 
The mouth makes its first appearance in Stage I as a row of 
dots lying in the middle line between the two mandibular 
arches (fig. 5), and connected by a kind of shallow groove in 
the ectoderm, along which the ectoderm and endoderm are 
fused. These pores soon become connected (fig. 6) to form a 
long slit, which extends from the ventral point of junction of 
the mandibular arches forward along the depression between 
the latter as far as the pocket of ectoderm which is destined to 
give rise to the pituitary body. The first rudiment of the 
mouth actually extends into the rudiment of the pitui- 
tary body. At the front end of the buccal slit the fore-gut, 
the notochord, the ectoderm, and the mesoderm are all con- 
