HUMAN EMBRYOS AND FETAL MEMBRANES 



flexed as in the earlier stage. Three branchial grooves separate the four 

 branchial arches. The first arch has developed two ventral processes. Of 

 these, the maxillary process is small and may be seen dorsal to the stomo- 

 dceum. 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 

 branchial groove may be seen the position of the oval otocyst, now a closed 



sac. Opposite the atrial portion 

 Mid-l 



Fore-brain 



StomodcRiim 



ind-brain 

 Otocyst 



Amnion (cut) 



of the heart, and in the region of 

 the caudal flexure, bud-like out- 

 growths 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 con- 

 strictions into four regions, or 

 vesicles, as in the fifty-hour chick 

 (Fig. 57). Of these, the most 

 cephalad is the telencephalon. It 

 is a paired outgrowth from the 

 fore-brain, the remaining 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 ecto- 

 dermal anlage of the lens. Its stage of development is between that of 

 the thirty-eight- and fifty-hour chick embryos. 



The otocyst is a closed sac, no longer connected with the outer ecto- 

 derm as in the fifty-hour chick. 



Digestive Canal. -In a reconstruction of the viscera viewed from the 

 right side (Fig. 86), the entire extent of the digestive canal may be seen. 

 The pharyngeal membrane, which we saw developed in the chick between 

 the stomodaeum and the pharynx, has broken through so that these cavities 

 are now in communication. The fore-gut, which extends from the oral 



Body stalk 



FIG. 85. Human embryo of 4.2 mm., in lateral 

 view (His). X 15- 



