336 



MISS E, A. FRASER ON THE DEVELOPMENT 



as their component cells are loosely connected and rather spread 

 out, so that they are not easily distinguished from the surrounding 

 mesenchyme. The primordium of the m. obliqnus superior is 

 moi-e compact than the rest and forms a large rounded mass above 

 the optic vesicle, posterior to and on the outer side of the supra- 

 orbital branch of the tiigeminal nerve. 



Perameles spp. (Text-fig. 25.) 



Our first stage of Perameles is an embryo of P. nasuta 

 measuring 5*7 mm. at its greatest length. The primary optic 

 vesicle is flattened and the outer layer thickened, and the 

 optic stalk contains a wide cavity communicating with the 

 fore-brain. The adjacent ectoderm is enlarged and slightly 

 invaginated to form the primordium of the lens. 



Text-figure 25. 



Perameles nasuta. G.L. 5"7 mm. 



Horizontal section tlirougli the head (S 4-1-11), showing the left head-cavity 

 (J.h.c.) when at its largest. X 110. 



a.c.a. = arteria cerebri anterior. FB. = fore-brain, lip. = hj'pophysis. 

 op.v. = optic cup. 



A small very reduced head-cavity is to be seen on each side 

 posterior and medial to the optic vesicle (text-fig. 25, l.h.c). 

 It is very irregular in form, is much broken up, and for the 

 most part is bordered simply by mesenchyme cells, a definite 



