THE EMBRYOLOGY OF THE OPOSSUM 73 



little way down the wall of the brain plate, whereas, the latter 

 arises from a more restricted region at the very edge of the 

 medullary plate. 



Miscellaneous details. No nephrocoele ever develops in the 

 opossum, so the myocoele is never connected with the splanch- 

 nocoele. Also the intermediate mesoderm does not become 

 segmented. The somites separate first from each other (i.e., 

 anteriorly and posteriorly), then from the intermediate meso- 

 derm (i.e., laterally). They then have the form of flattened 

 rectangular sacs enclosing a slit-like myocoele. 



The paired coelomic rudiments (i.e., splanchnocoeles) elon- 

 gate during stage 24 until they meet anteriorly and posteriorly, 

 thus surrounding the embryo. The anterior loop is seen in 

 figure 20, I, J and A. It extends beneath the subcephalic 

 wrinkle as the sectional view shows. However, instead of ex- 

 panding farther forward and thus eliminating the proamnion. 

 as is the case in other animals, in the opossum as the head 

 grows forward and the subcephalic wrinkle grows deeper, 

 this anterior loop of coelom is brought beneath the body and 

 in stage 25 it is simply incorporated into the definitive peri- 

 cardium, there being no extra-embryonic coelom anterior to 

 the embryo thereafter. This has an important effect upon the 

 constitution of the amnion, which will be discussed in 

 chapter X. 



The posterior loop of coelom remains largely extra- 

 embryonic. The anal plate dips in between it and the medul- 

 lary plate. Subsequently, as the caudal part of the body 

 rounds up, the proximal part of the extra-embryonic coelom 

 becomes ventral, enlarges, and receives the allantoic diver- 

 ticulum as that develops in stage 28 (fig. 37 and fig. 39, L). 

 The extra-embryonic somatopleure goes into the formation of 

 the caudal amniotic fold, and the splanchnopleure serves as 

 the bridge along which the vitelline artery passes to the area 

 vasculosa. 



The notochord in stage 23 was a broad plate in the endo- 

 derm except at its most posterior end where it was separate 

 from the endoderm and proliferating from Hensen's node. 



