142 



THE HUMAN EMBRYO. 



Human Embryo in the Tenth Stage with Four Gill Clefts Showing Externally. 



Although a few embryos belonging to this stage have been obtained, they 

 have yielded almost no satisfactory information. However, concerning em- 

 bryos slightly older, in which the posterior gill arches have begun to disappear 

 owing to the formation of the cervical sinus, we have quite accurate informa- 

 tion; hence, as we know the next younger and next older stages, we can form 

 fairly accurate conceptions of what must be the condition during the tenth 

 stage concerning which we lack direct observations. 



Cerv.s. 



al 



Op 



B.S 



Um. f. 



EMBRYO OF ABOUT TWENTY- 

 X about IO diams. (After 



FIG. 79. HUMAN 

 THREE DAYS. 

 W. His.} 



al, Anterior limb-bud. .S, Body stalk. Op, Optic 

 vesicle. //, Posterior limb bud. iv, Fourth 

 ventricle, i, Mandibular process. 2, Hyoid 

 arch, j, 4, Third and fourth gill arches. 



FIG. 80. HUMAN EMBRYO OF 7 MM. X 8 diams. 



(After f. P. Mall.} 

 a.l, Anterior limb bud. Cerv.s, Cervical sinus. A/of, 



Mandibular process. MX, Maxillary process. 



Na, Nasal pit. S, Eye. [/m.c, Umbilical cord. 



i, 2, First and second gill clefts. 



Human Embryo in the Eleventh Stage. Appearance of Limb=buds. Twenty= 

 three Days. 



In this stage the embryo is found very much rolled up, so that the head and 

 tail come very close together (Fig. 79) . It is further characterized by the appear- 

 ance of four protuberances, two upon each side of the body, in which we can 

 recognize the so-called limb-buds or anlages of the extremities. As shown in the 

 figure, the dorsal outline of the embryo describes more than a complete circle. 

 The embryo has a marked spiral twist, the head being bent to the right, the tail 

 to the left. The bending of the body is especially marked at the region of the 

 mid-brain or head bend, and at the posterior limit of the hind-brain. The anlages 

 of both pairs of limbs have appeared, but that of the leg is smaller than that of 



