THE ANATOMY OF A 4.2 MM. HUMAN EMBRYO 89 



body-stalk. There is an indication of the primitive segments along the dorso- 

 lateral line of the trunk. The head is bent ventrad almost at right angles 

 in the mid-brain region (cephalic flexure) . There are also marked cervical and 

 caudal flexures, the trunk ending in a short blunt tail. The heart is large and 

 flexed as in the earlier stage. Three gill clefts separate the four branchial arches. 

 The first has developed two ventral processes. Of these the maxillary process 

 is small and may be seen dorsal to the stomodaeum. The mandibular process is 

 large and has met its fellow of the right side to form the mandible or lower jaw. 

 Dorsal to the second gill cleft may be seen the position of the oval otocyst, now 

 a closed sac. Opposite the atrial portion of the heart and in the region of the 

 caudal flexure bud-like outgrowths indicate the anlages of the upper and lower 

 extremities. 



Central Nervous System and Sense Organs. — The neural tube is closed 

 throughout its extent and is differentiated into brain and spinal cord. The 

 brain tube or encephalon is divided by constrictions into four regions or vesicles 

 as in the fifty-hour chick (Fig. 55). Of these, the most cephalad is the telenceph- 

 alon. It is a paired outgrowth from the fore-brain, the persisting portion of 

 which is the diencephalon. The mid-brain or mesencephalon located at the 

 cephalic flexure is not subdivided. The hind-brain, or rhombencephalon, 

 which is long and continuous with the spinal cord, later is subdivided into the 

 metencephalon (region of the cerebellum and pons) and myelencephalon (medulla 

 oblongata). The spinal cord forms a closed tube extending from the brain to 

 the tail and containing the neural cavity, flattened from side to side. 



The eye is represented by the optic vesicles and the thickened ectodermal 

 anlage of the lens. Its stage of development is between that of the thirty-six 

 and fifty-hour chick embryos. 



The otocyst is a closed sac, no longer connected with the outer ectoderm as 

 in the fifty-hour chick. 



Digestive Canal. — In a reconstruction of the viscera viewed from the right 

 side (Fig. 81), the entire extent of the digestive canal may be seen. The pharyn- 

 geal membrane which we saw developed in the chick between the stomodaeum 

 and the pharynx has broken through so that these cavities are now in communi- 

 cation. The fore-gut, which extends from the oral cavity to the yolk-stalk is 

 differentiated into pharynx, trachea and lungs, esophagus and stomach, small 

 intestine and digestive glands (pancreas and liver). The gut is suspended from 

 the dorsal body wall by the dorsal mesentery. 



The ectodermal limits of the oral cavity are indicated dorsad by the diverti- 



