940 GENERATION. 



strual flow, at which an ovum is discharged has not yet been accurately determined ; and 

 it is subject, undoubtedly, to very considerable variations. It is certain, also, that ovu- 

 lation frequently does not take place until after the flow of blood has been established. 

 It is probable 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. If we admit, with Loewenhardt and Kundrat, that fecunda- 

 tion dates more nearly from a menstrual period prevented than from the last appear- 

 ance of the flow, it would be necessary to assume that ovulation usually takes place 

 before the flow, and fecundation would be most liable to follow intercourse occurring at 

 that time ; for we could hardly suppose 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 the uterus is prepared for its reception. In 

 a later memoir by Engelmann, published in 1875, he dissents from the view advanced 

 in the article published in connection with Kundrat, in 18T3, and states that he con- 

 siders the theory with regard to the temporal relations between menstruation and ovu- 

 lation to be " erroneous and wholly untenable." 



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

 nancy in individual cases, we must recognize the fact that the period is variable. Dat- 

 ing from the end of the last menstrual flow, we may adopt the average of 278 days, or a 

 little more than nine calendar months. 



Size, Weight, and Position of the Foetus. 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 th<? 

 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. 



