GERM LAYERS 39 



In young mammalian embryos the entire entoderm, with the noto- 

 chordal cells included in its dorsal part, forms the lining of a spherical 

 sac, known as the yolk-sac (Fig. 25, H). In birds the mass of yolk, 

 which may be regarded as lodged in the thickened ventral wall of the 

 yolk-sac, is so extensive that the cavity of the sac is merely a flattened 

 dorsal cleft. The yolk-sac gives rise to the entire intestinal tube, together 

 with all its outgrowths. They are therefore lined with entoderm, and 

 they develop as follows. 



At first, in the chick embryo (Figs. 27 and 28) a flattened finger-like 

 extension of the yolk-sac projects forward into the head, under the noto- 

 chord. This outpocketing is the fore-gut, which gives rise to the pharynx, 

 oesophagus, stomach, and anterior part of the small intestine. Near its 

 anterior extremity it comes in contact with the entoderm and fuses with 

 it, thus forming the oral membrane. By the rupture of this membrane, 

 an opening from the exterior into the pharynx is produced. 



Similarly the hind-gut develops as a pocket from the posterior part 

 of the yolk-sac. It gives rise to the lower portion of the small intestine 

 and the entire large intestine, and fuses with the ectoderm, forming the 

 cloacal membrane. In later stages the ventral part of the posterior 

 end of the hind-gut becomes cut off from the dorsal part; the ventral 

 subdivision forms the bladder, and the dorsal subdivision becomes the 

 lowest part of the rectum. At the same time the cloacal membrane is 

 correspondingly subdivided into the urogenital membrane which closes 

 the outlet of the bladder, and the anal membrane which closes the rectum. 

 Later these membranes rupture, and the line of separation between ecto- 

 derm and entoderm is then difficult to determine. The entoderm ap- 

 parently lines the entire urethra in the female, but only the upper or pro- 

 static portion in the male; the remainder is lined with ectoderm. 



In addition to forming the lining of the pharynx and entire digestive 

 tube, together with the bladder and its outlet, the entoderm lines the 

 following important organs, which arise as outgrowths of the pharynx and 

 digestive tube: the auditory tube, extending from the pharynx to the ear; 

 the thyreoid gland and certain constituents of the thymus; the entire 

 respiratory tract, including the larynx, trachea and lungs; the liver; and 

 the pancreas. 



Mesoderm. The mesoderm has already been described as forming 

 splanchnic and somatic layers. These are indicated in the diagram Fig. 

 25, H, but are more accurately shown in Fig. 29, which corresponds to 

 the upper part of Fig. 25, H, under higher magnification. Where the 

 somatic and splanchnic layers come together they are greatly thickened, 

 and the thickened part becomes cut into block-like masses by a series 

 of transverse clefts. The masses are called mesodermic somites, and a 

 pair of them occurs in each transverse segment of the body. They in- 



