982 CHRONOLOGY OF HUMAN DEVELOPMENT. 



extend in greater degree; the visceral clefts are closed with the exception of the 

 first; the allantois has but three vessels, as the right umbilical vein is obliterated. 

 In the fifth week the trunk is from 0.85 to 1.28 cm. long; the olfactory pits are 

 connected with the angles of the mouth by furrows, which close in the sixth week 

 and form canals. In embryos from thirty-five to forty- two days old the trunk 

 has a length of between i.i and 1.3 cm.; the buccal and nasal openings are sepa- 

 rate; the face is smooth; the extremities have three divisions; on the feet the 

 toes are not so well developed as the fingers. The auricle of the ear forms 

 first a low projection in the seventh week. The Wolfnan body is consider- 

 ably reduced. The length of the trunk at between the seventh and the eighth 

 week is from 1.6 to 2.1 cm. 



End of the Second Month. The ovum is 6 cm., the villi 1.3 mm. long; the 

 circulation in the umbilical vesicle is obliterated; the embryo is 26 mm. long and 

 weighs up to 4 gm. ; the eyelids and the nose are present. The umbilical cord is 

 8 cm. long; the abdominal cavity closed; ossification has begun in the lower jaw, 

 the clavicles, the ribs, the vertebrae; sex is undeterminable; the kidneys are 

 indicated. 



Third Month. The ovum is as large as a goose-egg; the formation of the 

 placenta has begun; the embryo (trunk-length between 2.1 and 6.8, total length 

 from 6 to 1 1 cm. ; weight 1 1 gm.) is from now on known as the fetus. The auricles 

 of the ear are developed; the umbilical cord is 7 cm. long; the external sexual 

 differentiation has begun; the navel is in the lower fourth of the linea alba. 



Fourth Month. The length of the fetal trunk is between 6.9 and 9 cm. ; the total 

 length from 10 to 17 cm.; the weight 57 gm.; the sex is distinct; hair and nails 

 have begun to form; the placenta weighs 80 gm.; the umbilical cord is 19 cm. 

 long; the navel is in the lower third of the linea alba; jerking movements of the 

 extremities occur; the bowels contain meconium; vessels are visible through 

 the translucent skin; the eyelids are closed. 



Fifth Month. The length of the fetal trunk is from 9.7 to 14.7, the total length 

 from 1 8 to 28 cm. ; the weight is up to 284 gm. The hair of the head and lanugo- 

 hairs are distinct; the skin is still somewhat light pink and thin, covered with 

 vernix caseosa, and not perfectly transparent. The weight of the placenta is 178 

 gm. ; the umbilical cord is 31 cm. long. 



Sixth Month. The length of the fetal trunk is from 15 to 18.7, the total length 

 from 26 to 37 cm.; the weight is 634 gm. The face acquires more fat and has a 

 less aged appearance ; the lanugo is thick and fluffy ; the amount of vernix is 

 increased; the testicles are in the abdomen; the pupillary membrane and the 

 eye-lashes are present; meconium is present down to the large intestine. 



Seventh Month. The length of the fetal trunk is from 18 to 22.8, the entire 

 length from 35 to 38 cm. ; the weight is 1218 gm. ; the large intestine has the same 

 length as the body (at an earlier date it is shorter, at a later date longer) ; the 

 descent of the testicles has begun, one testicle finding its way into the inguinal 

 canal; the eyes are open; the pupillary membrane often has disappeared at its 

 center in the twenty-eighth week; in addition to the primitive fissures, other 

 fissures begin to form. The fetus is viable. At the beginning of this month 

 there is a center in the os calcis. 



Eighth Month. The length of the fetal trunk is from 24 to 27.5, the total 

 length from 41 to 42 cm.; the weight is 1569 gm.; the hair of the head is thick, 

 13 cm. long; the nails have narrow margins; the navel is below the middle of 

 the linea alba; one testicle is in the scrotum. 



Ninth Month. The length of the fetal trunk is from 27 to 30, the total length 

 from 42 to 65 cm. ; the weight is 197 1 gm. ; the fetus at this age is not distinguish- 

 able from the mature fetus. 



Tenth Month. The length of the trunk is from 30 to 37, the total length from 

 45 to 67 cm.; the weight is 2334 gm. 



The Mature Fetus. The length of the body is 51 cm.; the weight 3 kilos 



(irom 2.5 to 5 kilos); lanugo-hair is still present only on the shoulders; the skin 



is white; the cartilages of the nose and the ears are hard to the touch. The 



nails project beyond the finger-tips. The navel is somewhat below the middle of 



the linea alba. The center of ossification in the lower epiphysis of the femur, 



vn 4 t0 t -L. mm ' m transverse diameter (it begins at the commencement or the 



ie of the ninth month, and is from 2 to 5 mm. wide at the end of the ninth 



nonth), is a characteristic of the mature fetus. There is often a center of ossi- 



ncation in the upper epiphysis of the tibia at the end of the tenth month. 



onclusion, the duration of development in the following animals will be 



