HISTORY OF THE EYE MUSCLES 



445 



the 'anterior' cavities are wanting (just as in some genera of Elas- 

 mobranchs), the preotic mesoderm shows a series of divisions, 

 the relations of which to adjacent organs are comparable with 

 those of VanWihje's first, second and third somites. These are 

 shown in text figures 10 and 11. If Koltzoff be correct in assert- 

 ing that the eye-muscles of Petromyzon are differentiated from 

 the walls of the first three mesodermic segments, their com- 

 parability with the first three somites of VanWihje is indisputable. 



4 i°'; 



Figs. 11, 12, 13 Semidiagrammatic camera drawings of sections of Petromy- 

 zon embryos, showing the mesodermic segmentation in the region of the head. 

 Figure 11 is from a parasagittal section of an 8-day embryo (Harv. embryol. coll. 

 A, sect. 35-36). The topographic relations (and their later development accord- 

 ing to Koltzoff '02) of the first three somites show them to be homologous with the 

 first three somites in elasmobranchs. 



Fig. 12 Cross section of an 8^day Petromyzon embryo through the region of 

 somite 2 (fig. 11). The similarity of the somite to the mesodermic pouches of 

 Amphioxus is obvious. 



Fig. 13- is a cross section of a slightly older Petromyzon embryo cut in the 

 region of the fourth somite, showing the differentiation of somatic and splanch- 

 nic mesoderm. 1, 2, 3, 4, Somites 1-4; cd.d. notochord; c«/, enteron;o<, auditory 

 placode; sp. spiracular pouch; spl, splanchnic mesoderm; Ih.n. neural tube. 



