270 EMBRYOLOGY 



villi. Embryonic blood vessels develop in the chorionic villi and are supplied 

 by the umbilical arteries and drained by the umbilical veins. The allantois, 

 which probably directs these blood vessels to the chorionic villi, develops as 

 an outgrowth of the posterior region of the endoderm of the yolk sac (Fig. 

 173). The endoderm itself becomes constricted into two parts by the forma- 

 tion of the umbilical cord, and the endoderm of the embryo proper remains 

 connected with the yolk sac by a thin strand of endoderm running through 

 the umbilical cord. 



The development of the embryo involves the formation of a primitive 

 streak which is very similar to that of the chick embryo. At the anterior end 

 of the primitive streak a head process forms, and above this head process the 

 neural plate develops (Fig. 174). This plate folds to form the primitive 

 neural tube. 



Comparison of chick and pig embryos 



Later development of a mammalian embryo follows closely the pattern 

 of the development of the chick embryo. Since pig embryos are readily avail- 

 able in large numbers, they form admirable material for a study of the de- 

 velopment of organ systems. Figure 175 shows a 10-mm. pig embryo with its 

 membranes removed and the body wall dissected to reveal internal structures. 

 This is somewhat more advanced than a 72-hour chick embryo. The 10-mm. 

 pig has well-developed limb buds by this time. The head and neck form a 

 curvature which brings the telencephalon close to the heart, as in the chick 

 embryo. A prominent tail is present, and the heart, liver, and mesonephros 

 can be seen. The development of the neural tube of the pig embryo is similar 

 to that of the chick, as shown in Figure 176. A similar section of the human 

 embryo is also shown for comparison. Note the differentiation of the motor 

 region within the spinal cord. The limb probably directs the nerve fibers in 

 mammals just as it does in the frog and the chick. There is no evidence of a 

 preformed pathway. Furthermore, observations on limbless individuals show 

 that the spinal ganglia and motor region of the cord are reduced in size and 

 thus confirm the idea that the limb controls differentiation of the motor 

 neurons in the spinal cord. 



The mammalian brain develops through the same stages as the chick 

 brain, as shown in Figure 177. The telencephalon, diencephalon, mesenceph- 

 alon, metencephalon, and myelencephalon are clearly marked out. Note 



