FETAL DEVELOPMENT 363 



rion and villi. Amnion folds. Umbilical vesicle partly 

 formed. Vascular area. Two primitive heart tubes. Gut 

 tract partly formed. 



Third Week. Body indicated. Dorsal outline concave. 

 Vitelline duct. Amnion. Allantoic stalk. Visceral arches. 

 Heart divides. Vitelline circulation begins. Gut tract still 

 connected with umbilical vesicle. Liver evagination begins. 

 Anal plate. Pulmonary protrusion. Wolffian bodies. Neu- 

 ral canal. The brain vesicles. Optic and otic vesicles. Ol- 

 factory plates. Notochord. 



Fourth Week. Flexion of body. Yolk-sac largest size. 

 Somites well formed. Allantois grows. Vitelline circulation 

 complete. Allantoic vessels developing. Pharynx, esopha- 

 gus, stomach and intestine differentiated. Pancreas begins. 

 Pulmonary protrusion bifurcates. Ventral roots of spinal 

 nerves. Limb buds apparent. 



Fifth Week. Umbilical vesicle begins to shrink. Cord 

 longer and spiral. Length of fetus two-fifths of an inch. 

 Primitive aorta divides into aorta and pulmonary artery. 

 Intestine shows loops. Bronchi divided. Ducts of Miiller. 

 Epidermis. Olfactory lobe. Eyes move forward. Limb 

 buds segment. Digitation indicated. 



Sixth Week. Umbilical vesicle shrunken. Amnion 

 larger. Vitelline circulation supplanted by allantoic^ Teeth 

 indicated. Duodenum, cecum. Rectum. Larynx. Genital 

 folds and ridges. Dorsal roots of spinal nerves. Eye-lids. 

 Lower jaw and clavicle begin to ossify. Vertebrae and ribs 

 cartilaginous. Fingers separate. 



Seventh Week. Body and limbs well defined. Heart 

 septa complete. Transverse and descending colon. Nails 

 indicated. Cerebellum indicated. Muscles recognizable. 

 Ossification in cranium and vertebrae begins. 



Eighth Week. Hiead somewhat elevated. Parotid gland. 

 Gall bladder. Miillerian ducts unite. Genital groove. Mam- 

 mary glands begin. Sympathetic nerves. Nose discernible. 

 Additional centers of ossification. 



