HEAD CAVITIES AND NERVES OF ELASMOBRANCHS, 70 
celomic cavity left between the two layers of the mesoblast 
further back. 
In the anterior part of the head the dorsal ends of the head 
cavities are situated above the tops of the visceral clefts (vide 
fig. 2) ; consequently, after these clefts are formed, the several 
head cavities, while they are separate from one another below, still 
communicate together dorsally. At the commencement of stage 1 
the dorsal ends of the three anterior head cavities still com- 
municate together, but between stages 1 and Kk they become 
separated. ‘This point, which is not noticed by Balfour, appears 
to be one of some importance, inasmuch as we have in this 
division of the dorsal part of the head cavity a segmentation of 
the mesoblast of the head, which is not directly caused by the 
visceral clefts, although it takes place along the same lines, or, 
more strictly, along dorsal prolongations of these lines, 
The dorsal portions of the first three head cavities lying above 
the level of the tops of the visceral clefts (vide Plate V, fig. 9, 
1, 2, 3) become, at a still later stage, cut cff from the ventral 
portions lying in the visceral arches. The subsequent changes 
undergone by these dorsal and ventral portions differ mate- 
rially from one another, as will be noticed later on. In the 
trunk we also find a division of the coelomic cavity on either 
side into a dorsai or vertebral portion, which forms a series of 
cavities occupying the centres of the protovertebre, and a ven- 
tral or parietal portion forming the peritoneal cavity. 
It becomes now an interesting question, which, owing to 
insufficient material, Iam unable as yet to answer definitely, 
whether this division of the head ccelom into dorsal and ventral 
portions is not strictly comparable to the division of the body 
cceelom into vertebral and parietal portions. I have only 
observed these dorsal portions in the first three head cavities— 
the premandibular, mandibular, and hyoidean cavities. 
“ Between stages 1 and k the anterior (premandibular) cavi- 
ties of the two sides are prolonged ventralwards, and meet below 
the base of the fore brain. The connection between the two 
cavities appears to last for a considerable time, and still persists 
at the close of stage u.”1 This median communication between 
the two premandibular cavities is shown at stage L in Plate V, 
fig. 13,1. Fig. 14 represents a longitudinal and vertical section 
very near to the middle line, and shows the premandibular cavity 
(1), still of considerable size. Fig. 13 represents the median 
section from the same embryo; at 1 is seen the median com- 
munication between the two premandibular cavities ; this is small, 
but has a perfectly distinct and obvious lumen. This median 
portion of the first head cavity has interesting relations to the 
; ' Balfour, op. cit., p. 207, 
