278 



EMBRYOLOGY 



Fig. 186. The embryo at about one month old. (From Julius Kollman, 

 Handatlas der Entwkklun gs geschicht e des Menschen. Verlag von Gustav Fischer, 

 Jena.) 



and the otocyst is pear-shaped. The neural folds have closed completely, and 

 a well-developed brain has formed. The heart shows the same spiral arrange- 

 ment as in the chick embryo. 



At four weeks the embryo is about 5 mm. long and weighs about 20 mg. 

 (Fig. 186). At this stage 40 somites are present, the arm and leg buds have 

 formed, and the embryo corresponds roughly to about a three- to four-day 

 chick embryo. The neural tube is completely closed now, and the divisions of 

 the brain are more sharply defined. The optic primordia have become optic 

 cups, and the lens is forming. The auditory placode has formed the otocyst, 

 and the olfactory placodes are present as thickenings of the head ectoderm. 

 In the nervous system nerves are beginning to develop in the motor region 

 of the spinal cord and from the spinal ganglia. In the region of the pharynx 

 the endoderm has formed five visceral pouches and the primordia of the 

 tongue are present. The laryngotracheal groove has developed into paired 

 lungs and trachea, and the thyroid sacs are now well defined. The liver is 

 clearly developed, with its ducts and with the primordium for the gall 

 bladder. The pancreas is represented by diverticula from the midgut. There 

 is a clear differentiation between the esophagus, stomach, and intestine. 



