92 GENEEAL HISTOEY OF DEVELOPMENT 



The tail, which began to disappear in the sixth week, is still distinct at the 

 of the second month, but reduced to a minute tubercle. The human characl 

 are all established, and the embryo may now be spoken of as the foetus. 



Third month. Fig. 128 shows the general relations of foetus and uterus at 

 the beginning of the third month. The decidua capsularis is not yet fused with 

 the decidua vera. It is still a thickish layer intimately associated with the villi 

 of the chorion Iseve. The amnion fills the whole chorionic vesicle. The umbilical 

 cord is a short, stout, and now twisted structure. The foetus by the end of the 

 month measures about 7 cm. from vertex to coccyx. All its parts have nearly 

 assumed their relative proportions, though the head is still large. The eyelids 

 and lips are closed, and the auricle folded. The nails have appeared on fingers 

 and toes, and the external genital organs are apparent. The vitelline loop of the 

 intestine is now withdrawn into the body- cavity. 



Fourth month (fig. 129). During the fourth month the foetus increases to 

 12 cm. or 13 cm. in length from vertex to coccyx. The muscles are now so 

 far developed as to give rise to movements of the limbs and body. 



The skin becomes firmer, and is rose-coloured. Short colourless hairs appear 

 on the head, and finer downy hairs over the rest of the body. The chin is a more 

 prominent feature, the arms and legs are of nearly equal length, the umbilicus is 

 situated close above the pubes, and the sex characters are fully established. 



Fifth month. By the end of the fifth month the foetus measures about 

 20 cm. from vertex to coccyx, and 25 cm. to 27 cm. if the legs be included in 

 the measurement. Its weight is now about half a kilogramme. The skin shows 

 patches of sebaceous matter, and the hair is better developed. The legs are longer 

 than the arms, and the umbilicus lies farther forwards. 



Sixth month. At the end of the month the length of the foetus from vertex 

 to heels is 30 cm. to 32 cm. ; it has doubled its weight, which is now about one 

 kilogramme. The skin is wrinkled and dull red in colour. Sebaceous matter has 

 increased, especially in the axillae and groins. The hair is darker and stronger, 

 and the eyebrows and eyelashes appear. The umbilicus is still farther forwards. 



Seventh month. The length of the foetus at the end of the month, from 

 vertex to heels, is about 35 or 36 cm. ; it weighs about 1J- kilogrammes. Owing 

 to the deposit of subcutaneous fat, the body has become plumper ; the eyelids 

 re-open, and the hair is now plentiful on the head. The foetus, if born at this period, 

 is capable of surviving. 



Eighth month. During the eighth month the foetus increases to 40 to 45 cm. 

 in length from vertex to heels, but the increase in bulk is more marked ; it now 

 weighs from 2 to 2J kilogrammes. The skin loses the dull red tint, and becomes 

 of a bright flesh -colour. Its surface is covered all over with sebaceous matter, 

 now known as the vernix caseosa : the layer is thickest on the head and in the 

 axillae and groins. 



Ninth month. At birth the average length of the foetus is about 50 cm. 

 from vertex to heels, that is, about 20 inches; the average weight is about 

 3 to 3J kilogrammes, or 6| to 7J pounds. The skin is paler than in the eighth 

 month, the body is more plump and rounded ; the hair is long and abundant on 

 the head, but the downy hair (lanugo) on the body has begun to disappear. The 

 umbilicus now occupies the centre of the body. 1 



1 For literature other and later than His's Anatomie menschlichen Embryonen, Leipzig, 1885, and 

 Archiv Anat. u. Phys. Anat. Abt., 1892, see Keibel in Hertwig's Handbuch, I. Teil i., ii. 174 ; and general 

 literature list, I. Teil i. 83 ; P. Michaelis, Arch. Gyniikol. Ixxviii. ; Retzius, Bi'ol. Untersuchungen, 

 N.F. xi., Stockholm, 1904. Kollmann (Handatlas Appendix, p. 37) gives a full list of papers. In 

 addition are the following : Gage, Amer. Jour. Anat. iv. ; Bremner, Amer. Jour. Anat. v. ; Bonnot and 

 Seevers, Anat. Anzeiger, xxix. ; P. Thompson, Jour. Anat. Phys. xli. ; Ingalls, Arch. mikr. Anat. Ixx. 



