REPRODUCTION 133 



all; they are always separated by one or more tissues. However, sub- 

 stances can diffuse, or be transported by some active process, from mother 

 to fetus or the reverse. 



The form of the placenta, and the intimacy of the connection be- 

 tween maternal and fetal tissues, varies from one mammal to another. 

 The placenta of the pig or cow has scattered villi over the chorionic 

 surface and is said to be diffuse. The chorionic villi of the placenta of 

 carnivores occur in a cylindrical band around the chorion; this is known 

 as a zonary placenta. The primate and rodent placenta is disc-shaped 

 and is called a discoidal placenta. The number of layers of tissues that 

 intervene between maternal and fetal blood vessels varies from 2 in the 

 man and rat to 6 in the sheep. 



The giowth of the embryo and of the amnion brings the edges of 

 the amniotic folds together to form a tube which encloses the yolk sac 

 (which is usually small or vestigial), the allantois, the two umbilical 

 arteries and the umbilical vein which pass to the placenta. This tube, 

 the umbilical cord, is composed of a peculiar, jelly-like material which 

 is unique to the cord. 



The anmion, chorion and allantois, together with the egg shell or 

 placenta, are adaptations which permit the embryos of the higher verte- 

 brates to develop on land; they are a substitute for the pond or sea water 

 in which the embryos of the lower vertebrates develop. 



39. The Control of Development 



Biologists have been interested for many years in the nature of the 

 factors which regulate the complex, orderly processes leading to the 

 production of a new adult from a fertilized egg. How can a single cell 

 give rise to many different types of cells, which differ widely in their 

 morphologic, functional and chemical properties? 



Early embryologists believed that the egg or the sperm contained a 

 completely formed but minute germ which simply grew and expanded 

 to give the adult. This preformation theory explained development by 

 denying that it occurred! An extension of this theory postulated that 

 each germ contained within it the germs for all succeeding generations, 

 each within the next. Some microscopists reported seeing this germ 

 within the sperm or egg and described the "homunculus," a fully formed 

 little man inside the egg or sperm! Others calculated the number of 

 germs that were present in the ovaries of Eve, the mother of the human 

 race, and suggested that when all of these were used up the human race 

 would end. 



The contrasting theory of epigenesis, first advanced by Wolff in 1759, 

 stated that the unfertilized egg is not organized and that development 

 involves progressive differentiation which is controlled by some outside 

 force. VV^e now know that development is not simply epigenetic, for there 

 are certain potentialities localized in particular regions of the egg and 

 the early embryo. The embryos of certain species, when separated into 

 parts at an early stage, will develop normally; each part forms a com- 

 plete, normal, though small, embryo. The embryos of other species show 



