940 GENERATION. 



has not been accurately determined ; and it is certain that ovulation frequently does not 

 take place until after the flow of blood has been established. This question we have 

 fully considered in a previous chapter. It is probable, also, that intercourse is most liable 

 to be followed by fecundation, when it occurs just after the cessation of a menstrual 

 period, and when the female often presents unusual sexual excitement; but it is true that 

 fecundation may follow intercourse at any time. If we admit that fecundation dates 

 more nearly from a menstrual period prevented than from the last appearance of the 

 flow, it would be necessary to assume that ovulation usually takes place before tbe flow, 

 and fecundation would be most liable to follow intercourse occurring at that time; for 

 we could hardly admit that an ovum, fecundated at the cessation of a menstrual period, 

 could remain in the generative passage of the female for two or three weeks, before the 

 mucous membrane of tbe uterus is prepared for its reception. Tbese facts are so strong, 

 that the view entertained by Kundrat and Engelmann cannot yet be adopted without 

 reserve. 



As regards the practical applications of calculations of the probable duration of preg- 

 nancy in individual cases, we must recognize the fact that the duration is variable. If 

 we date from the end of the last menstrual period, we may adopt the average of 278 

 days, a little more than nine calendar months. If we adopt the view that pregnancy 

 dates from a menstrual period which has been prevented, the duration of intra-uterine 

 life would be about 250 days. 



Size, Weight, and Position of the Fffitus. The estimates of writers with regard to the 

 size and weight of the embryon and foetus at different stages of intra-uterine life present 

 very wide variations ; still, it is important to have an approximate idea, at least, upon 

 these points, and we shall adopt the figures given by Scanzoni, as presenting fair aver- 

 ages. As the measurements and weights are simply approximative, the slight differ- 

 ences between the German and the English standards are not important. It will be 

 useful, also, to give, as is done by Scanzoni, a review of the general development of the 

 organs at different stages. 



At the third week, the embryon is from two to three lines 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. 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 gen- 

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



At the eighth week, the embryon is from ten to fifteen lines 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 abdomi- 

 nal walls have closed over in front. 



At the third month, the embryon is from two to two and a half inches long and 

 weighs about one ounce. The amniotic 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 various 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 from 

 four to five inches long and weighs about five ounces. The muscles begin to manifest 

 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 from nine to twelve inches long and weighs from 

 five to nine ounces. 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. 



