792 HANDBOOK OF PHYSIOLOGY. 



In this cartilaginous rod three centres of ossification appear : basi- 

 occipital, basi-sphenoid, and pre-sphenoid, one corresponding to each 

 segment. 



The bones forming the vault of the skull, viz., the frontal, parietal, 

 squamous portion of temporal and the squaino-occipital, are ossified in 

 membrane. 



The Visceral Clefts and Arches. 



As the embryo enlarges, the heart, which at first occupied a position 

 close to the cranial flexure, is carried further and further backward until a 

 considerable part, in which the mesoblast is undivided, intervenes between 



Pig. 486. A. Magnified view from before of the head and neck of a hurnan embryo of about 

 three weeks (from Ecker.) 1, anterior cerebral vesicle or cerebrum; 2, middle ditto; 3, middle 

 or fronto-nasal process; 4, superior maxillary process; 5, eye; 6. inferior maxillary process, or 

 first visceral arch, and below it the first cleft ; 7, 8, 9, second, third, and fourth arches and clefts. 

 B. Anterior view of the head of a human foetus of about the fifth week (from Ecker, as before, 

 fig. IV.). 1, 2, 3, 5, the same parts as in A; 4, the external nasal or lateral frontal process: 6, 

 the superior maxillary process; 7, the lower jaw; X, the tongue; 8, first branchial cleft becom- 

 ing the meatus auditorius externus. 



it and the head. This becomes the neck. On section it is seen that in 

 it the whole three layers are represented in order, and that there is no 

 interval between them. In the neck thus formed soon appear the vis- 

 ceral or branchial clefts on either side, in series, across the axis of 

 the gut not quite at right angles. They are four in number, the most 

 anterior being first found. At their edges the hypoblast and their 

 epiblast are continuous. The anterior border of each cleft forms a fold 

 or lip, the branchial or visceral fold. The posterior border of the last 

 cleft is also formed into a fold, so that there are four clefts and five folds, 

 but the three most anterior are far more prominent than the others, and 

 of these the second is the most conspicuous. The first fold nearly meets its 

 fellow in the middle line, the second less nearly, and the others in order 

 still less so. Thus in the neck there is a triangular interval, into which 

 by the splitting of the mesoblast at that part the pleuroperitoneal cavity 

 extends. The branchial clefts and arches are not all permanent. The 

 first arch gives off a branch from its front edge, which passes forward to 

 meet its fellow, but these offshoots do not quite meet, being separated 



