306 



MISS E. A. FRASER ON THE DEYELOPMENT 



with the maxillo-mandibuhir mesenchyme. On its anterior side 

 this group of looser cells, to which in following stages I have given 

 the name of the intermediate mass and which represents the second 

 or mandibular somite, projects slightly forwards anterior to the 

 cavity ; this projection, as we shall see later, is the first indication 

 of the primordium of the m. obliquus sviperior. The intermediate 

 mass is, at the same time, connected with the postero-lateral 

 wall of the cavity for three or four sections. The limits of 

 the complex formed by the intermediate and abducens masses 

 are difficult to determine accurately, as they are not easily 

 distinguished from the surrounding mesenchyme and in embryo 

 (a) are still more indefinite. In the longitudinal series, a distinct 

 but small elongated collection of cells stretches along the posterior 

 border of the cavity, apparently representing the abducens mass, 

 but no connection Avith any maxillo-mandibular mesenchyme is 

 recognisable. 



Text-figure 3 







a c a 



''/ 



Trichasuriis vulpecitla. G.L. 4'5 mm. (/3 '98). 



Transverse section (S 3-6-13) passing thvongli the premandibular liead-cavity on 

 each side. Tho sections are rather oblique, tlie cavit.y of the left side being cnt 

 nearer its ventral end. The anterior portion of the abducens muscle-mass lies 

 posterior to the cavity. X 110 and reduced by |-. 



«.c.«. = arteria cerebri anterior. ^"-5.= fore-brain. lJi.e.=Mt head-cavity. op.v.= 

 optic vesicle. )'.e.= anterior portion of abducens muscle-mass. r.)i.e.=right 

 head-cavity. v.c.Hj.^vena capitis medialis. v.o.i.=\enA orbitalis interior. 



The Gasserian ganglion is large and the mandibular branch of 

 the trigeminal nerve is already well developed. The oculomotor 

 nerve runs from the mid-brain anteriorly for a short distance but 

 does not reach the head-cavity. 



