302 MISS E. A. FRASER ON THE DEVELOPMENT 



arch (see his fig. V, p. 139, Marcus, '09). The premandibnlar 

 cavity, which is small and has also in early stages a connecting 

 median portion, partly degenerates, but a small part moves up to 

 the mandibular cavity and completely vinites with the rostral side 

 of the latter. All the eye muscles, except the m. rectus externus 

 derived from the third somite, take their origin from the 

 mandibular cavity which is innei'vated by the oculomotor and 

 perhaps also by the trochlear nerve. Marcus ('10) further states 

 (p. 409) : — " Daher glaube ich, das, wenn auch Priimandibulax- 

 zellen sich mit der Mandibularmasse vereinigt haben, nicht 

 deswegen der Oculomotorius die Mandibularhohle innervirt, 

 sondern dass er ganz primar der zngehorige ISTerv ist. Dies 

 ergiebt sich, ausser durch die naive Betrachtung, auch durch 

 folgende Ueberlegung. Aus dem rostralsten Teil der Mandibu- 

 larhohle entwickelt sich der Muse, obliquus superior, also ein vom 

 Trochlearis innervierter Muskel. Die Zellmasse, woraus er sich 

 entwickelt, glaube ich aus der Pramandibularmasse ableiten zu 

 konnen. Exackt beweisen kann ich diese Behauptung freilich 

 nicht, weil bei der Muskelbildung die Mandibularhohle schon 

 mesenchymatos war, also die Muskelbildung nicht so scharf wie 

 bei Selachiern verfolgt werden kann ; aber aus der Topographie 

 der Gebilde ergibt es sich, das zur Bildung des Muse, obliquus 

 superior wie bei Selachiern so auch bei Hypogeophis dieser 

 Abschnitt der Mandibularhohle verwendet wird." 



Although many authors have given a,ccounts of the origin of 

 these preotic somites and of the first development of the eye 

 muscles from their walls, few have traced in detail the further 

 growth of these muscles until their adult position is reached. 

 In reptiles and mammals this latter growth is more difficult to 

 follow owing to the development of the m. retractor bulbi. The 

 most complete account in reptiles is that of Ghehjdra serpentitia 

 by C. E. Johnson ('13), and the only description in mammals is 

 that of Renter's paper ('97) on the pig, above mentioned. The 

 following observations on the marsupial Trichoswus vulpexula 

 will therefore be of some interest. 



. Material. 



The material examined comprises twelve stages of Trichosurus 

 vulpecula, eight embryos and four pouch young of Phascolarctos 

 cinereus, five embryos of Phascolomys mitchelli, an embryo and a 

 foetus of Macropus, five embi-yos and one pouch young of 

 Perameles, one embryo of Didelphys and a good series of 

 Dasyiirus. 



Two wax-plate models of the optic cup and primordiaof the eve 

 muscles were made and are reproduced on Plates I. & II. I am 

 much indebted to Miss E. A. Steele for her excellent coloured 

 drawings of these models. The terminology of the arteries and 

 veins in the head has been adopted from Grosser's account of the 

 development of the vascular system in the Chiroptera ('01) and 



