REPRODUCTION AND DEVELOPMENT 63 



has been primarily responsible for this change in shape. Observe the 

 primordia of five digits in the paddle-like limb buds. Recall that the 

 pig at birth has only two fully differentiated digits and two that are 

 rudimentary on each foot. How do you think this embryonic recapit- 

 ulation of the typical vertebrate plan of appendage can be explained? 

 Draw (X 6) an embryo as it lies on its side. 



Exercise 74. — The Development of the Face. 



(m) Since the ventral surface of the head is so closely pressed 

 against the trunk, changes that occur can be seen only in heads re- 

 moved from the trunk. Examine a demonstration of heads taken 

 from embryos of approximately 6-, 10-, 15-, and 20-mm. crown- 

 rump length. These heads should be observed from the ventral view. 

 Notice the formation of the lower and upper jaws, the appearance of 

 the cerebral hemispheres as seen through the head covering, the shift- 

 ing contours of the region which change the position of the external 

 nares, and the beginning of the elongation of the snout. The early 

 molding of the facial region is brought about by proliferation of 

 mesenchyme between the skin and the brain. 



Exercise 75. — The Reproductive System of a Pregnant Sow. 



(n) Examine a demonstration of the reproductive system w'hich 

 has been removed from a sow in early pregnancy. Notice the ovaries 

 in which prominent corpora lutea and smaller follicles can be seen. 

 The oviducts are not paired throughout their lengths as in the frog but 

 are fused at their posterior ends to form the cervix, or neck, of the 

 uterus which has two horns, each differentiated from one of the ovi- 

 ducts. Near the ovaries the oviducts are very small and are known 

 as the Fallopian tubes, each of which has a funnel-shaped opening, or 

 ostium, which closely surrounds the ovary. Eggs when shed from the 

 ovaries pass directly into the Fallopian tubes. 



(o) Depending on the stage of pregnancy the horns of the uterus 

 may exhibit local enlargements. Note in one horn which has been 

 opened the very elongated chorionic vesicles, each containing an em- 

 bryo and its membranes; the covering of the chorionic vesicle is the 

 outermost embryonic membrane, the chorion. The conspicuously 

 vascularized allantois by its extensive growth is responsible for the 

 dilation of the vesicle but does not extend to its ends. 



(p) In an opened chorionic vesicle the chorio-allantoic membrane 

 has been cut, and the delicate amnion surrounding the embryo can be 

 seen. The amniotic cavity between the amnion and the embryo is 

 filled with amniotic fluid in life. What is the function of this fluid? 

 The belly stalk can perhaps be observed through the amnion. 



