340 MISS E. A. FRASER ON THE DEVELOPMENT 



stage or may disappear before the cavities have a-ttained their 

 maximum size. This variation is mentioned by Corning ('00) 

 (p. 66) : — " Aus den Oppel'schen Figuren, wie aus den meinigen, 

 geht hervor, wie stark die Variationen sind, welche man in 

 Bezug auf die Aiisbildnng des mittleren Yerbindungsstiickes 

 zwischen den beiden Kopfhohlen antrifi"t. Nicht selten bleibt 

 die Verbindung noch in relativ spater Zeit bestehen, in anderen 

 Fallen is sie schon zu einer Zeit verschwunden, wo die Hohlen- 

 bildung im lateralen Theile noch nicht auf ihrem Hohepunkt 

 angelangt ist, in noch anderen sehen wir im Stiel einzelne 

 kleinere Hohlenbildungen auftreten, die sich spater mehr Oder 

 weniger voUstandig zu einer grossen Hohle vereinigen." 



In the 5 mm. embryo of Trichosurus and in the 6-7 mm. 

 Macropus, the small median portion partially constricted off from 

 the rest of the cavity (text-fig. 1, n.) may possibly coincide with 

 the swollen part of the stalk, which runs from the somite to the 

 middle line in Atiiguisfragilis (called by Oppel the "Hals") and 

 which takes a part in the formation of the head -cavity. 



We have seen that the walls of the premandibnlar cavity, as 

 in most other Vertebrates, give rise to the muscles innervated 

 by the oculomotor nerve. In Trichosurus and probably also in 

 Phascolomys, the primordium of the m. obliquus inferior and the 

 common primordium of the mm. rectus inferior and rectus 

 internus develop as solid outgrowths from the posterior and 

 ventral walls, whilst the primordium of the m. rectus superior 

 arises from the walls of a hollow evagination on the dorsolateral 

 side, the latter mode of origin resembling that of all the oculo- 

 motor muscles in Lacerta (Corning, '"99). In Phascolarctos, 

 elongated hollow outgrowths occur along the greater part of the 

 posterior wall of the cavity, an extensive budding here taking 

 place. The further development of these three muscles in the 

 marsupials agrees very closely with that of the same muscles in 

 Chehjdra (Johnson, '13). 



The m. rectus internus and the m, rectus inferior develop from 

 a common primordium, the m. rectus internus first appearing as 

 an oflshoot from near the proximal end of the m. rectus inferior; 

 this offshoot grows directly upwards and outwards to the anterior 

 side of the bulbus. In Chehjdra, according to Johnson ('13), the 

 common primordium becomes transformed into " a solid elongate 

 mass. By the 11 mm. stage a constriction has appeared, slightly 

 beyond the middle of this mass, differentiating it into a proximal 



M. rectus inferior and a distal M. rectus medialis The 



proximal end, which at first is continuous with the distal end of 

 the M. rectus inferior, works up along the medial side of this 

 muscle so that the final separation of the two takes jjlace at 

 their proximal ends, i. e. their ends of origin." No movement of 

 this kind has been observed in Trichosurtts, and if it occurs it 

 must take place in the short interval between the 10 mm. and 

 11 mm. stao'es. 



