622 
MR. W. K. PARKER OK THE STRUCTURE AND 
membrane being the inner wall of the body-cavity, or “ splanchnopleure the “sornato- 
pleure ” of this part is not developed quite so far down towards the ventral line. 
The space between the pericardium and outer (costal) wall is only part of the general 
“ pleuro-peritoneal ” space or cavity; with this all are familiar. 
But it is not generally known that the divisions of the mesoblast of the head 
acquire a cavity on each side. “ These cavities end in front, opposite the blind 
anterior extremity of the alimentary canal; behind, they are continuous with the 
general body-cavity. I propose to call them head-cavities. The cavities of the two 
sides have no communication with each other.” (Balfour, £ Elasmobranchs,’ p. 206 .) 
For the sake of those who have not Mr. Balfour’s work at hand, I will give, below, 
a continuation of his excellent description of these cavities.'"' 
In both Stage 2 (Plate 37, fig. 2) and in the younger specimens of Stage 3 (Plate 37, 
figs. 4, 5) I succeeded in making thick slices from the side of the head, showing 
three cavities ( h.c .). It will be seen that the one close to the eye ( mx.p ., h.c 1 .) is 
widest towards the mid-line of the head, and that the others are widest outside, and 
narrow towards the ventral fine. All the post-oral cavities are distinct from each 
other, right and left; but the pre-oral cavity (“premandibular ” of Balfour —I should 
call it the palatine head-cavity) opens into its fellow of the opposite side. I find the 
head-cavities in embryos of Chelone viriclis at this same stage. I may also refer to the 
looped appearance of the optic nerve (fig. 4, II), and to the deep folds of the hind 
brain, in front of the oval, nut-like ear-sac (fig. 5, C 3 , au.) 
* “ Coincidently with the formation of an outgrowth from the throat to form the first visceral cleft, 
the head-cavity on each side becomes divided into a section in front of the cleft and a section behind the 
cleft (vide plate 14, fig. 3, b, and 6, jp.p.) ; and during stage H it becomes, owing to the formation of a 
second cleft, divided into three sections ; (1) a section in front of the first or hyo-mandibular cleft; 
(2) a section in the hyoid arch between the hyo-mandibular cleft and the hyo-branchial or first branchial 
cleft; (3) a section behind the first branchial cleft. 
“ The section in front of the hyo-mandibular cleft stands in a peculiar relation to the two branches of the 
fifth nerve. The ophthalmic branch of the fifth lies close to the outer side of its anterior part, the man¬ 
dibular branch close to the outer side of its posterior part. During stage I this front section of the 
head-cavity grows forward, and becomes divided without the intervention of a visceral cleft, into an 
anterior and posterior division. The anterior lies close to the eye, and in front of the commencing mouth 
involution, and is connected with the ophthalmic branch of the fifth nerve. The posterior part lies 
completely within the mandibular arch, and is closely connected with the mandibular division of the 
fifth nerve. As the rudiments of the visceral clefts are formed, the posterior part of the head cavity 
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.”—(p. 206.) 
Again (p. 207) we read that “ the anterior or premandibular pair of cavities are the only parts of the 
body-cavity within the head that unite ventrally.” 
When Mr. Balfour says that the foremost part of the cavity becomes “ divided, without the inter¬ 
vention of a visceral cleft, with an anterior and posterior division,” he evidently rejects the mouth-angles 
from the category of clefts. I do not think that the ophthalmic branch of the fifth nerve is the proper 
segmental nerve of the maxillo-palatine fold. 
