616 J. T. PATTERSON 



to find a common canal for the two embryos developing out of 

 the proximal part of the primary bud. 



Each embryonic rudiment is a slipper-shaped structure lying 

 at the terminus of a canal (fig. 5), and is in a relatively late 

 primitive streak stage. At the anterior end of the embryo the 

 medullary plate is well formed, although as yet the medullary 

 folds have not become elevated. Posteriorly the embryo ends 

 as an irregular, blunt process of mesodermal tissue. 



Fig. 78 shows a transverse section of one of the embryonic 

 tubes from specimen No. 276. The section passes through the 

 embryo at the level of the extreme anterior tip of the primitive 

 streak, and hence cuts the thickened medullary plate, which 

 curves upward and inward to become the thin ectodermal layer 

 of the amnion. Beneath the medullary plate, and between it and 

 the entoderm, are seen the scattered mesodermal cells which 

 have been proliferated from the primitive streak. 



Lateral to the embryo is a loose mass of mesodermal cells 

 which lie between the entoderm and epithelial-like mesoderm 

 of the extraembryonic cavity. In the whole condition of the 

 chorionic vesicle the mesoderm was seen to fringe each side of 

 the embryonic tube, extending throughout the entire length of 

 the embryo proper. It already shows a rudimentary net-work 

 of blood-vessels, which represents the beginning of the area vas- 

 culosa. I have not worked out the detailed history of the vas- 

 cular area, but undoubtedly it arises in each embryo in a man- 

 ner similar to that in the typical mammalian ovum from which 

 but a single embryo develops. 



The embryonic mesoderm is thickest at the posterior end of 

 the embryo, where it gives rise to a series of enlargements which 

 extend for some little distance behind the extreme posterior tip 

 of the embryonic tube. This is the portion of the mesoderm 

 into which the umbilical vessels and the rudimentary divertic- 

 ulum of the allantois later extend; that is, it forms the basis 

 for the belly-stalk. 



The further development of each embryo is very similar to 

 that of the ordinary mammal, and therefore calls for a brief 

 description only. The reader is referred to an earlier paper *in 



