FCETAL LIFE 851 



calculation most in use by obstetricians is to date from the end of the 

 last menstrual period. Taking into account, however, the various cases 

 quoted by authors, in which conception has been supposed to follow a 

 single coitus, there appears to be a range of variation in the duration of 

 pregnancy of not less than forty days, the extremes being two hundred 

 and sixty and three hundred days. As regards the practical applica- 

 tions of calculations of the probable duration of pregnancy in individual 

 cases, the fact must be recognized that the period is variable. Dating 

 from the end of the last menstrual flow, an average of two hundred and 

 seventy-eight days, or a little more than nine calendar months, may be 

 adopted. 



Size, Weight and Position of the Fcetus. Estimates in regard to -the 

 size and weight of the embryo and foetus at different stages of intra- 

 uterine life present wide variations ; still, it is important to have at least 

 an approximate idea as to these points, and the estimates by Scanzoni 

 are given, as presenting fair averages. 



At the third week the embryo is two to three lines (4.2 to 6.4 

 millimeters) in length. This is about the earliest period at which 

 measurements have been taken in the normal state. 



At the seventh week the embryo measures about nine lines (19.1 

 millimeters). Points of ossification have appeared in the clavicle and 

 the lower jaw ; the Wolffian bodies are large ; the pedicle of the umbili- 

 cal vesicle is much reduced in size ; the internal organs of generation 

 have just appeared ; the liver is of large size ; the lungs present several 

 lobules. 



At the eighth week the embryo is ten to fifteen lines (21.2 to 31.8 

 millimeters) in length. The lungs begin to receive a small quantity of 

 blood from the pulmonary arteries ; the external organs of generation 

 have appeared, but it is difficult to determine the sex ; the abdominal 

 walls have closed over in front. 



At the third month the embryo is two to two and a half inches 

 (50.8 to 63.5 millimeters) long and weighs about one ounce (28. 3 grams). 

 The amniotic liquid is then more abundant in proportion to the size of 

 the embryo than at any other period ; the umbilical cord begins to be 

 twisted ; the abdominal glandular organs appear ; the pupillary mem- 

 brane is formed ; the limitation of the placenta has become distinct. 

 At this time the upper part of the embryo is relatively much larger than 

 the lower portion. 



At the end of the fourth month the embryo is called the foetus. 1 It 



1 The periods of evolution are divided differently by authors. His distinguishes the three 

 periods, as follows : First two weeks, the product is called the ovum ; from the third to the fifth 

 week, the embryo ; and after the fifth week it is called the foetus. 



