66 HUMAN ANATOMY. 



extra-foetal portion of the intestinal canal, which at an earlier period passes into 

 the umbilical cord, during the fourth month recedes within the abdomen. The 

 differentiation of sex is still more sharply exhibited by the external organs : in the 

 male the penis is acquiring a prepuce, and in the female the labia majora and the 

 clitoris are becoming well developed. At the close of this period the foetus measures 

 approximately 150 millimetres and weighs about 120 grammes. 



During the fifth month the first foetal movements are usually observed. The 

 heart and the liver are relatively of large size. The decidua capsularis fuses with 

 the decidua vera, thereby obliterating the remains of the uterine cavity. The meco- 

 nium within the intestinal canal shows traces of bile. The advent of the fine hair, 

 the la)iugo, first upon the forehead and the eyebrows, and somewhat later upon 

 the scalp and some other parts of the body, represents a conspicuous advance. 

 Likewise adipose tissue appears in places within the subcutaneous layer. The 

 approximate length, at the end of the fifth month, is twenty-three centimetres 

 and the average weight about 320 grammes. 



The sixth month is characterized by complete investment of the body by 

 lanugo and by the appearance of the vernix caseosa, the protecting sebaceous secre- 

 tion which coats the body of the foetus to prevent as far as possible maceration of 

 the epidermis in the amniotic fluid. The latter now reaches the maximum quantity, 

 being contained within the large sac of the amnion. The sixth month is distin- 

 guished by the conspicuous increase both in the size and weight of the foetus, and 

 is known, therefore, as the period of greatest absolute growth. At the close of the 

 sixth month the foetus measures approximately thirty-four centimetres in its longest 

 dimension and weighs about 980 grammes. 



The seventh month is marked by progressive changes in the various parts 

 of the foetus, whereby the more advanced details become pronounced in the central 

 nervous system and digestive tract. The length of the foetus at the close of the 

 seventh month approximates forty centimetres and its weight about 1700 grammes. 



The eighth month is occupied by the continued growth and general develop- 

 ment, as part of which the foetus acquires greater plumpness than before and a 

 brighter hue of the integument, now entirely covered with vernix caseosa. The 

 lanugo begins to disappear, while the scalp is plentifully supplied with hair ; the nails 

 have reached, or project beyond, the tips of the fingers. By the close of the eighth 

 month the foetus has attained a length of about forty-six centimetres and a weight 

 of about 2400 grammes. 



The ninth month witnesses the gradual assumption of the characteristics of 

 the child at birth, among which are the rounder contours, the extensive, although 

 not complete, disappearance of the lanugo, except from the face, where it largely 

 persists throughout life, the completed descent of the testicles within the scrotum, 

 the approximation of the labia majora, the permanent separation of the eyelids, with 

 well-developed lashes, and the presence of dark greenish meconium within the in- 

 testinal canal. The umbilicus has reached a position almost exactly in the middle 

 of the body. The average length of the foetus at birth is about fifty centimetres, 

 or twenty inches ; its average weight, while included between widely varying 

 extremes, may be assumed as approximately 3100 grammes, or 6.8 pounds. The 

 weight of the foetus at term is materially influenced by the age of the mother, 

 women of about thirty-five years giving birth to the heaviest children. The weight 

 and stature of the mother probably also affect the weight of the child. Repeated 

 pregnancies exert a pronounced effect upon the foetus, since the weight of the child 

 reaches the maximum with the fifth gestation. 



The purpose of the preceding pages is to present an outline of the general 

 developmental processes leading to the differentiation and establishment of the defi- 

 nite body-form of the human embryo ; a more detailed account of the development 

 of the various parts of the body is given in connection with the descriptions of the 

 systems and the individual organs, to which the reader is referred. 



