62 THE FROG . 



hind limb bud the tail will be seen. Draw (X 15) an embryo as it lies 

 on its side. 



(j) The cut edges of the amnion and yolk sac will probably be 

 found where they are attached to the embryo along the mid-ventral 

 region of the trunk. The amnion, chorion, and allantois of the pig 

 embryo are formed in the same way as in the chick but at a relatively 

 earlier stage of development. The yolk sac is really formed at the 

 gastrula stage of development but not distinctly separated from the 

 gut until somewhat later. However, since the mammalian egg is not 

 yolky the chief nutritive, as well as respiratory and excretory, func- 

 tions are performed by the allantois in close association with the 

 chorion. This chorio-allantoic membrane is closely applied to, but 

 not fused with, the lining of the uterus. The association of embryonic 

 membranes and the tissue of the mother constitutes a placenta by 

 means of which exchange of products dissolved in the blood of either 

 can occur by diffusion between mother and embryo. 



Exercise 73. — Older Pig Embryos. 



(k) A pig embryo of 9-12 mm. in length is about three weeks old 

 and corresponds in stage of development to a human embryo of the 

 same length which is about six weeks old. The features described for 

 the 6-mm. embryo should be identified. The eye is now more easily 

 seen and the beginning of the external ear can be observed. Note that 

 the trunk of the embryo is somewhat straighter along the mid-dorsal 

 line. This straightening is brought about by increased growth of the 

 internal organs especially the heart, liver, and mesonephroi. The 

 belly stalk, which corresponds to the umbilical cord of a human 

 embryo, contains the stalks of the yolk sac and allantois which are 

 connected to the gut of the embryo; part of the amnion forms the 

 covering of the belly stalk. Draw ( X 10) an embryo as it lies on its 

 side. 



(1) Pig embryos 15-20 mm. in length are 24-28 days old, that is, 

 they have completed about one-fourth of their embryonic life. A 

 human embryo of 17-18 mm. is about seven weeks old and is compar- 

 able to the pig in stage of development. In a pig embryo of this 

 length note the external naris, the eye with its lids, and the external 

 ear which is the only remaining trace of the grooves posterior to the 

 mouth. It is still possible to identify the thin roof of the myelen- 

 cephalon and a thicker ridge that is the roof of the metencephalon just 

 anterior to it. Notice that, while the flexures in the head region per- 

 sist, the trunk is much straighter dorsally than in the last stage. The 

 very extensive growth of the liver, two lobes of which may be seen. 



