8o 



THE FETAL MEMBRANES AND EAELY HUMAN EMBRYOS 



MiJ-hrain 



Fore-brain 



StomodcBum 



Mandibular 

 process 



Heart 



llind-hrii'm 



Aiii]il{irv vesical 



I'ranchial 

 arches 



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 stomodaum. 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-hke 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. 57). Of these, the most ceph- 

 alad is the telencephalon. 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 subdi\'ided into the 

 metencephalon (region of the cere- 

 , bellum 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. 86), the entire extent of the digestive canal may be seen. The pharyn- 

 geal membrane, which we saw developed in the cliick between the stomodseum 



I IN n ion {cut) 



Fig. 85.- 



Bodv stall; 



-Left side of a human embryo of 4.2 mm. 

 (His). X 15. 



