74 PROFESSOR A. MILNES MARSHALL. 
show that the cavities at this stage extend forwards beyond the 
anterior end of the alimentary canal and end rather abruptly 
immediately behind the outgrowths of the fore brain that give 
origin to the optic vesicles. I have not found either at this or 
any other stage any trace of a head cavity in front of the optic 
vesicles. The cavities of the two sides are at stage H quite 
distinct from one another, though their walls are very close 
together anteriorly. 
During stage @ a pair of lateral diverticula arise from the 
alimentary canal, and form the rudiments of the first pair of 
visceral clefts—the spiracular or hyomandibular clefts. These 
diverticula, as they increase in size, first press the two layers of 
mesoblast together, so as to obliterate the head cavities opposite 
their points of impact, and then gradually displace the mesoblast, 
the hypoblast of the diverticula ultimately coming into contact 
with the external epiblast. After a short pause perforation of 
the epiblast is effected, and the visceral cleft is completed. In 
this way the head cavity on either side becomes divided into a 
part in front of the hyomandibular cleft, and a part behind this 
cleft. 
Balfour’s account of the succeeding stages, which my own 
observations simply confirm, is as follows :!—“ During stage 1 
this front section of the head cavity (the part in front of the 
hyomandibular cleft) grows forward and becomes divided, without 
the intervention of a visceral cleft, into an anterior and a poste- 
rior division. ‘The anterior lies close to the eye and in front of 
the commencing mouth involution..... The posterior part lies 
completely within the mandibular arch..... As the rudiments 
of the successive visceral clefts are formed the posterior part of 
the head cavity (behind the hyomandibular cleft) becomes 
divided into successive sections, there being one section for each 
arch. Thus, the whole head cavity becomes, on each side, 
divided into—(1) a premandibular section; (2) a mandibular 
section; (3) a hyoid section; (4) sections in the branchial 
arches.” 
The obliteration of the head cavity by the rudimentary visceral 
clefts is well shown in Plate V, fig. 3, which represents a 
somewhat oblique section, passing through the hind brain (A. 4.), 
auditory pits (avd.), and third visceral (second branchial) clefts 
(a/’) of an embryo of stage 1; 4.c. is the portion of the head 
cavity left between the second and third visceral clefts, ¢.e. in 
the first branchial arch ; a/’ is the diverticulum of the alimentary 
canal which forms the rudiment of the third visceral cleft, oppo- 
site which the two layers of mesoblast are seen to be brought 
into contact so as to obliterate the head cavity ; and 4.c. is the 
1 Op. cit., p. 206, 
