THE ARCHENTERON. 



57 



enlarged, first, by the withdrawal of the notochordal cells from the vertebrae into the 

 adjacent intervertebral discs; second, by the growth of the tissue. The cavities 

 occupied by the nuclei have distinct boundaries and present characteristic forms in 

 different mammals. The sheath of the notochord is lost, the walls of the cells dis- 

 appear, the tissue becomes a syncytium (Fig. 24) of granular appearance, and 

 breaks up into multinucleated reticular masses, 

 making an irregular network the meshes of which 

 are filled with a more or less homogeneous sub- 

 stance resembling mucin, that does not, however, 

 agree with mucin in its reactions. Tissue of this 

 character may be easily observed in human embryos 

 of the third and fourth month. It has been not 

 infrequently stated that the notochord disappears in 

 mammals, and that it contributes to the formation 

 of cartilage. Both statements are now known to 

 be erroneous. Owing to the persistence of the 

 nucleus pulposus, the vertebral joint differs funda- 

 mentally from all other joints in the body of the adult. 



FIG. 24. PIG EMBRYO OF 150 MM. 

 Notochordal syncytium from nucleus 

 pulposus. X 800 diams. (After L. 

 W. Williams.) 



The Archenteron. 



The archenteron comprises the entire cavity bounded by the entoderm. At 

 first it consists chiefly of the cavity of the yolk-sac (Fig. 25), but as it also in- 

 cludes the embryonic entodermal tract its development in the embryo greatly pre- 

 dominates as growth continues. As the head of the embryo protrudes, the archenteron 

 forms a cephalic prolongation, known as the fore-gut (Figs. 25, Vd, and 132, Vd), which 

 ends blindly in front, but opens behind (caudad) into the general archenteric space, 

 its opening being termed the fovea cardiaca, fo. Later as the caudal region becomes 

 protuberant the archenteron sends into it a similar blind prolongation, known as 

 the hind-gut (Fig. 25, H.g). .As the embryo grows compare the section on growth, 

 page 49 the connection between the embryo and the yolk-sac, which seems so 

 large in early stages (Fig. 25), increases very little, and therefore becomes relatively 

 smaller. It never attains more than 3 or 4 mm. The embryo, on the contrary, 

 grows enormously (Fig. 34), and there is a corresponding enormous lengthening of 

 the fore-gut and hind-gut. The former is the anlage of the pharynx, oesophagus, 

 and stomach. The latter is the anlage of the large intestine and most of 

 the ileum. 



During embryonic life the archenteron is divided by the obliteration of the con- 

 nection between the yolk-sac and the embryonic entoderm. For a time the ori- 

 ginal point of connection is marked by a small pouch (MeckeVs diverticulum) of the 

 ileum. The pouch normally disappears, but as an occasional anomaly (arrest of 

 development) it persists in the adult. 



