OF THE EYE MUSCLES IN MARSUPIALS. 341 



The abclucens complex and the m. obliquus superior are 

 difficult to identify in our earliest stages, but by comparison 

 with slightly older embryos it is possible to make out their 

 contours in the surroundixrg mesenchyme from which they are 

 not easily distinguished. In our first stages, up to 8-5 mm., 

 the primordia of both these muscles are united with the maxillo- 

 mandibular mesenchyme by an intermediate mass of more loosely 

 connected cells, the position of which is well seen in Phascolomys 

 (text-fig. 21, p. 332). The m. obliquus superior arises from this 

 intermediate mass as an upgrowth which extends forwards above 

 the eye, the intermediate mass itself apparently degenerating. 

 In the rabbit, Edgeworth ('03) regards the m, obliquus superior 

 and the m. rectus externus as "specialised portions of the man- 

 dibular and hyoid myotomes which separate, the former late, the 

 latter very early in development, from the upper ends of their 

 respective myotomes " (p. 82). In Chelydra, according to Johnson 

 ('13), the m. obliquus superior (p. 159) "grows forward as a stream 

 of cells from the dorsal portion of the mesenchymal cell-mass 

 which results from the second head somite," the ventral portion 

 of the latter at the 5 mm. stage being in close contact with the 

 mesoderm of the mandibular arch. If we compare these con- 

 ditions with those in Trichosuribs, Phascolarctos and Phascolomys, 

 we may conclude with some probability that the intermediate 

 mass answers to the second somite of the head whose cavity is 

 already obliterated, or in which a cavity has never developed, 

 and from whose dorsal region the m. obliquus superior takes its 

 origin. In Chelydra also, at a certain stage in development, the 

 identification of the second somite is a matter of some ditficulty. 

 In the embryo of 7 mm. Johnson says (p. 142) : — "The second 

 head somite of the 7 mm. stage is of such indistinct and indefinite 

 form that it may easily escape notice. It reaches here the most 

 obscure phase of its development. The more or less conspicuous 

 cavities of earlier stages have col]a|3sed and broken down, and 

 with their disappearance the cells of their walls are with difficulty 

 distinguished from the intruding and intermingling mesenchymal 

 elements." 



No cavity is seen in the abducens muscle-mass, which in our 

 earliest embryo is quite solid and shows a temporaiy attachment, 

 as above mentioned, to the intermediate mass, this connection 

 being probably a secondary phenomenon as in Chelydra, where it 

 also occurs. It is possible that the intermediate mass, 2. e. the 

 second somite, may contribute towards the formation of the 

 m. rectus externus as in some fishes (Dohiii, '04, Neal, '14), but 

 we have no direct evidence of this in Trichosurus. The m. rectus 

 externus and m. reti-actor bulbi develop exactly as in Chelydra ; 

 the origin of the m. retractor bulbi bearing no resemblance to 

 that of the pig where, according to Renter ('97), p. 376 : — 

 " Dieser Muskel ensteht aus dem inneren Mantel des Augen- 

 muskelkelches durch Abspaltung von vorn nach hinten." 



