SIZE, WEIGHT AND POSITION OF THE FCETUS. 843 



cases which are quoted by authors, in which conception has been supposed to 

 follow a single coitus, there appears to be a range of variation in the dura- 

 tion of pregnancy of not less than 40 days, the extremes being 260 and 300 

 days. As regards the practical applications 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 278 days, or a little more than nine calendar months, may be 

 adopted. 



Size, Weight and Position of the Fcetus. The estimates of writers with 

 regard to the size and weight of the embryon and foetus at different stages of 

 intrauterine life present very wide variations ; still it is important to have 

 an approximate idea, at least, upon these points, and the estimates by Scan- 

 zoni are given, as presenting fair averages. 



At the third week the embryon is two to three lines (4-2 to 6-4 mm.) in 

 length. This is about the earliest period at which measurements have been 

 taken in the normal state. 



At the seventh week the embryon measures about nine lines (19-1 mm.). 

 Points of ossification have appeared in the clavicle and the lower jaw ; the 

 Wolffian bodies are large ; the pedicle of the umbilical vesicle is very 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 embryon is ten to fifteen lines (21-2 to 31 -8 mm.) 

 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 embryon is two to two and a half inches (50-8 

 to 63-5 mm.) long and weighs about one ounce (28'3 grammes). The amni- 

 otic fluid is then more abundant, in proportion to the size of the embryon, 

 than at any other period ; the umbilical cord begins to be twisted ; the vari- 

 ous glandular organs of the abdomen appear ; the pupillary membrane is 

 formed ; the limitation of the placenta has become distinct. At this time 

 the upper part of the embryon is relatively much larger than the lower 

 portion. 



At the end of the fourth month the embryon becomes the foetus. It is 

 then four to five inches (10-1 to 12-7 centimetres) long and weighs about 

 five ounces (141-7 grammes). The muscles begin to show contractility ; 

 the eyes, mouth and nose are closed ; the gall-bladder is just developed ; the 

 fontanelles and sutures are wide. 



At the fifth month the foetus is nine to twelve inches (22-8 to 30-5 centi- 

 metres) long and weighs five to nine ounces (141-7 to 255-1 grammes). The 

 hairs begin to appear on the head ; the liver begins to secrete bile, and the 

 meconium appears in the intestinal canal ; the amnion is in contact with the 

 chorion. 



At the sixth month the foetus is eleven to fourteen inches (27'9 to 35-5 

 centimetres) long and weighs one and a half to two pounds (680 to 907 



