348 H. B, POLLARD. 



The bridge of mesoderm still persists behind the infundibulum 

 (fig. 3) in such a manner as to lead one to suppose that 

 formerly the eye muscles extended across the middle line, 

 possibly when no cartilaginous skeleton was as yet formed. 



The pericardial cavities have extended to the middle line 

 and fused, and the heart is formed, though as yet solid (fig. 3). 



The development of the pericardial walls in Teleostei seems 

 to me to have an important bearing on some theories of the 

 Vertebrate head. According to Van Wijhe (24), in Selachii 

 the cavities of the visceral arches, whose walls give rise to the 

 muscles of the gills and jaws, are continuous with the pericardial 

 cavity, but not (at least in the posterior arches) with the 

 cavities of the somites. Therefore, concludes Van Wijhe, the 

 muscles of the gills and jaws, which are voluntary, are homo- 

 logous with the muscles of the intestine, which are involuntary, 

 and not with the body muscles, which are voluntary. Van 

 Wijhe further makes this one of the cardinal points for the 

 interpretation of the nerves. In Teleostei the somites are, as 

 is well known, solid, and the ccelom with its walls, the 

 somatopleure and splanchnopleure, is sharply defined. Now 

 the coelom in Teleostei is perfectly continuous far forward 

 (fig. 2), but its walls, the somatopleure and splanch- 

 nopleure, the " Seitenplatten" of Van Wijhe, do not 

 give rise to the gill and jaw muscles. Yet it does not 

 seem possible to deny the general homology of the gill and jaw 

 muscles in Selachii and Teleostei. 



The Jaws. 

 Fig. 11 shows the first condensation of mesoderm around 

 the mouth, and it is seen that one Anlage gives rise to 

 skeletal elements of both upper and lower jaws. The 

 condensation of mesoderm extends forward above theectodermic 

 ingrowth of the mouth some little distance below the eye. 

 Below the mouth the lower jaw at this stage extends only a 

 very slight distance forward, though further towards the 

 middle line. In fig. 3 the structures are represented schema- 

 tically. The dotted line extending forward below the eye 



