316 HUMAN PHYSIOLOGY 



Fourth month. Length of foetus 17 cm, weight 100 g. It is 

 possible to distinguish male and female organs from each other. 

 Placenta weighs 80 g. First movements of the extremities. Mcco- 

 nium in intestine. 



Fifth month. Length of foetus 30 cm, weight 280 g. Hair on 

 the head and body [lanigo] appear. Beginning of sebaceous secre- 

 tions. Placenta weighs 178 g. 



Sixth month. Length of foetus 34 cm, weight 700 g. The fat 

 layers of the skin develop. Movement of the embryo. Born dur- 

 ing this month the child makes feeble respiratory movements but is 

 not viable. 



Seventh month. Length of foetus 38 cm, weight 1300 g. Born 

 during this month the child whines and is sometimes viable. 



Eighth month. Length of foetus 42 cm, weight i57og. Tes- 

 ticles descend. Child is viable. 



Ninth month. Length of foetus 65 cm, weight 1970 g. 



The mature embryo is 50 cm long, weighs 3 kg. 



2. METABOLISM OF THE EMBRYO 



(a) Circulation. In explaining the embryonic circulation 

 two periods must be kept distinct: (i) the period of vitel- 

 line [yolk-sac] circulation ; (2) period of the placental cir- 

 culation. 



(i) Vitelline circulation. The first formation of vessels 

 occurs near the germinal disk. From the cells of the 

 mesoderm originate the peripheral veins (sinus terminal is) 

 from which spring the blood vessels of the embryo. From 

 the wall of the vein solid cords of cells extend into the 

 embryo which anastomose and become hollow by the forma- 

 tion of intercellular spaces filled with an intercellular fluid. 

 The heart is formed from two symmetrical vessels in the 

 alimentary canal in the head which represent the primitive 

 aortae. These coalesce in the median line, forming a tube. 

 From this tube the heart is developed by the formation of 

 an S-like curve, whereby the tube is divided into an auricle, 

 ventricle, and truncus arteriosus. By a partition appearing 

 in the tube, the right and left heart are formed. From the 

 heart there spring originally two aortic arches which give 

 off the omphalo-mesenteric arteries. The branches of these 

 arteries pass through the germinal disk to the sinus ter- 



