PHYSIOLOGY OF THE EMBRYO 315 



lying below the germinal disk, i.e. the embryonic intestine, 

 from the part above, i.e. the yolk-sac. The connection 

 between these two cavities, as long as it is open, is called the 

 vitelline duct (see Fig. 49). 



The ectoderm forms a fold over the curved germinal disk. 

 The inner leaf of this fold grows over the embryo and forms, 

 by separating from the blastula, the amnion, which at the 

 navel passes over into the skin of the embryo. The outer 

 leaf of the fold joins the zona pellucida and forms with it the 

 serous membrane, which later on is called the chorion. On 

 the surface of the chorion villus-like processes are formed 

 which unite with the mucous membrane of the uterus. From 

 the posterior part of the embryonic intestinal cavity a 

 tubular projection, the allantois or the urinary sac, grows 

 out into the space between the yolk-sac and the chorion. 

 Its inner part (lying in the embryo) becomes the urinary 

 bladder. The allantois grows outward until, in the third 

 week, it joins the chorion and forms with it the placenta. 

 The lumen of the allantois soon disappears and forms a cord 

 composed of mucous tissue and is called the umbilical cord. 



Chronology of the development of the embryo. 



First month. 



First week. Passage of the egg through the tube ; fertilization ; 

 formation of the blastula. 



Second week. Blastula attains a diameter of 5 mm ; villus-like 

 processes formed on the egg membrane ; first rudiments of embryo ; 

 formation of the spinal folds and of the medullary groove. 



Third week. Embryo about 4 mm long. Formation of the 

 amnion, yolk-sac, and allantois. The yolk-sac circulation is 

 established. 



Fourth week. Embryo from 8 to 1 1 mm long. The position of 

 the extremities is clearly visible. The three cerebral vesicles are 

 present. 



Second month. Length of embryo 30 mm. The yolk-sac circu- 

 lation degenerates, while the placental circulation develops. Forma- 

 tion of the face ; disappearance of the gill-clefts and posterior gill 

 arches ; the extremities become jointed ; first points of ossification in 

 the hip-bone and lower jaw ; abdominal cavity closed ; kidneys 

 formed. 



Third month. Length of embryo 70 mm. Commencement of 

 sexual differentiation. 



