224 ESSENTIALS OF ZOOLOGY 



others, las for example the pig, they separate and only the embryonic 

 portion is shed at time of birth. In the other forms, some of the 

 uterine portion is shed along with the embryonic, and together they 

 are known as ** afterbirth. " 



The circulation of blood in the placenta is by distinct maternal and 

 embryonic vessels which do not join each other. The materials in 

 solution in the blood of either can reach the other by diffusion only, 

 as the vessels and sinuses come in intimate contact in the membrane. 

 The embryo then does not receive the mother's blood as such, because 

 there i* not a direct connection between the two systems. The em- 

 bryo, or later the fetus, receives its nourishment and oxygen through 

 diffusion in the placenta and disposes of carbon dioxide and nitrog- 

 enous wastes by similar diffusion in the opposite direction. 



The time elapsing from time of conception, or fertilization, to time 

 of birth is called the gestation period. In the human being this 

 period is of nine months' duration. 



In the female mammal there is a definite cycle of changes which 

 occur in the uterus, in other sexual organs, and in physiologic rela- 

 tions of the body. This cycle is called the oestrus cycle. In the 

 human being the period required for this complete cycle averages 

 twenty-eight days. At one point in this cycle there occurs rather 

 severe degeneration of the uterine lining and some bleeding. This 

 period in the cycle is known as menstruation period. It was thought 

 earlier that the ovum was discharged from the ovary at this time and 

 entered the oviduct, but at present it seems to be more commonly 

 accepted that ovulation occurs during the 13th to 15th day following 

 the onset of menstruation. The length of the oestrus cycle varies 

 widely in different mammals. In most of them the female will mate 

 only at a certain period in the cycle. This is designated in common 

 terms as the ''heat" period. 



Biogenetic Law, or Theory of Recapitulation 



It is almost impossible to study the embryology of vertebrate 

 animals of the different classes without observing some features of 

 the apparent progressive development in regard to particular or- 

 gans and systems. In a number of respects the more highly de- 

 veloped forms seem to recapitulate or pass through the stages Avhich 

 existed through the long history of their ancestors. The appearance 

 of gill arches and even part of the slits in certain stages in the em- 

 bryonic development of mammals indicates that these animals pass 



