362 REPRODUCTION 



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. Neural 

 canal. The brain vesicles. Optic and otic vesicles. Olfactory 

 plates. Notochord. 



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

 Somites well formed. Allantois grows. Vitelline circulation 

 complete. Allantoic vessels developing. Pharynx, esophagus, 

 stomach and intestine differentiated. Pancreas begins. Pul- 

 monary 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. Intes- 

 tine shows loops. Bronchi divided. Ducts of Miiller. Epider- 

 mis. Olfactory lobe. Eyes move forward. Limb buds seg- 

 ment. 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. Head somewhat elevated. Parotid gland. 

 Gail bladder. Mullerian ducts unite. Genital groove. Mam- 



