92 GENERAL SKETCH OF 



Branchial system. 



In the early embryo the hypoblast forms the roof of the 

 sub-germinal cavity, which has inferiorly the periblast envelop- 

 ing the yolk. Behind and below the ears a fold grows up on 

 each side of the middle line, meeting with the epiblast on each 

 side of the ears. The lower edges of the lateral folds approach 

 each other in the mid-line, fuse and close in the branchial 

 region of the gut (Wilson). The same takes place with the 

 trunk behind, but Cunningham denies this. Behind and below 

 the • ears a large oval area is apparently pushed in, resulting 

 in the perforation of the epiblast on each side. 



The formation of the primitive operculum is not readily 

 understood, for the oesophageal lumen is not yet formed. The 

 opercular flap is a much later outgrowth from the tympanic 

 region — which grows backwards over the gill-slits. Below the 

 hind-brain and ears the hypoblast shows a great increase of 

 cells, and beneath them mesoblastic cells make their way, 

 bounded below by periblast, and these become columnar, 

 forming paired rod-like masses. 



Some days before emerging from the egg three branchial 

 bars are usually visible, a long thread-like lacunar vessel is 

 found along the posterior margin of each bar, and later a 

 septum forms — constituting an upper arterial and a lower 

 venous trunk. 



Five transverse bands, sometimes indications of six, extend 

 across the floor and under the flattened oesophagus, forming 

 a series of mesoblastic cross ridges, and between these the 

 hypoblast is pushed in as septa. The hypoblastic diverticula 

 indicate the future position of the gill-openings, while the 

 mesoblastic margin forms the gill-arches and branchial skeleton. 

 The fifth branchial arch remains rudimentary in the forms dealt 

 with here. In front of the four branchial arches are the stout 

 bars of the hyoid arch, and again in front of these is the 

 mandibular arch with cartilage-cells which are readily dis- 

 tinguished by their height and stoutness. The elements 

 connected with the suspensory apparatus of the lower jaw and 

 the skeleton of the jaws gradually develop. In the young cod 



