HEAD SOMITES AND EYE MUSCLES IN CHELYDRA 123 



Hoffmann was at a loss as to how to interpret these two masses, 

 but he stated that it was conceivable that the anterior one cor- 

 responded to the third head somite of selachians, the posterior 

 mass to the fourth. As opposed to this he found that the latter 

 did not occupy a position above the second gill pouch and below 

 the auditory vesicle, as does the fourth head somite in selachians, 

 but that it lies above the first gill cleft. Furthermore, in sela- 

 chians the M. rectus lateralis is developed from the third head 

 somite and is innervated by the N. abducens. In lizards another 

 muscle is also innervated by this nerve, namely the M. retractor 

 oculi. This muscle is not found in selachians. It is probable 

 therefore, according to Hoffmann, that both cell-masses belong 

 to the third head somite and that the anterior mass gives rise 

 to the M. rectvis lateralis and the posterior mass produces the 

 M. retractor oculi. This he was unable to verify for lack of 

 material. 



The head somites of Anguis fragilis were investigated by Oppel 

 ('90). The youngest embryo studied by him was one of 11 

 segments. At this stage, at the place where Hoffmann found 

 the first head somites of Lacerta, Oppel describes his observa- 

 tions in effect as follows: From the point where the anterior, 

 blind end of the foregut abuts against the floor of the forebrain 

 the mesoderm extends from the midline laterally into the head, 

 on both sides. In front of this point there is no mesoderm. A 

 part of the laterally extending mesoderm is noticeable as being 

 sharply differentiated from the rest. This part grows out later- 

 ally from the midline, gradually broadening, and extends anter- 

 iorly toward the eye. It has the form of two wings attached 

 at a common point. These mesodermic wings lie behind the 

 optic vesicles, only slightly separated from them, and, curving 

 around them laterally and ventrally, they extend still further 

 forward. The connecting-bridge ('Verbindungs-briicke') of these 

 two wing-like structures, which at the same time forms the point 

 at which the chorda and gut-wall meet, has a posterior protrud- 

 ing thickening or process, in which the end of the chorda disap- 

 pears. Oppel calls this structure uniting the organs mentioned 

 the prechordal plate ('Praechordalplatte'). The foregut touches 



