76 



HUMAN EMBEYOLOGY, 



(Mall). Forty-three mesodermal somites are present, but only about twenty-one are visible 

 on the surface. During the fifth week the lens of the eye appears as a thickening of the 

 surface ectoderm ; sinks into the interior of the eyeball ; becomes a vesicle and separates 

 from the surface. The three segments of the fore-limb become visible, and the rudiments 

 of the fingers appear. The hind-limb is less advanced ; the thigh segment is not distinct, 

 and the rudiments of the toes are not yet visible. The third arid fourth visceral arches 

 disappear from the surface and lie in the depths of the precervical sinus, a depression 

 between the neck and the anterior part of the body ; this is overlapped, superficially, by 

 the caudal margin of the second arch, which grows tailwards and forms the operculum of 



FIG. 97. KRCEMER - PFANNENSTIEL Zr- 

 GOTE. (From Keibel and Elze's 

 Normaltafdn. ) The embryonic 

 region is folded into the form of an 

 embryo, which is 1 '9 mm. long, and 

 it is possibly about three weeks old. 

 At the lower end of the Fig. (the 

 caudal end of the embryo) are seen 

 portions of the chorion and body- 

 stalk. The cerebral portion of the 

 neural rudiment is defined. Six 

 pairs of mesodermal somites are 

 present, but there are no signs of 

 limbs. 



FIG. 98. BALLE'S EMBRYO. 

 (From Keibel and Elze's 

 JVormaltafeln.) Length after 

 hardening in alcohol 2*5 

 mm. The neural groove 

 is closed from the sixth 

 somite to within a short 

 distance of the caudal end, 

 but it is open anteriorly. 

 The hind-, mid-, and fore- 

 brain regions and the optic 

 vesicle can be distinguished. 

 At the lower end of the 

 Fig. is the body-stalk, and 

 at the right side a part of 

 the yolk-sac. 



FIG. 99. PFANNENSTIEL'S EM- 

 BRYO. (From Keibel and 

 Elze's Nornudtafeln.} Length 

 of embryo about 2 '6 mm. 

 The rudiment of the otic 

 vesicle is seen in the Fig. 

 above the second branchial 

 cleft. The heart and peri- 

 cardium from the bulging 

 eminence below the head and 

 the Wolffian ridge is seen at 

 the lateral border of the meso- 

 dermal somites. 



the sinus (Figs. 101, 102). During the fifth week the head grows rapidly, and becomes 

 relatively very large as contrasted with the body. 



During this week also the olfactory pits appear, and grow dorsally in the roof of the 

 stomatodseum, separating the median from the lateral nasal processes ; the median process 

 is divided into the two globular processes ; and the maxillary processes of the mandibular 

 arches, growing towards the median plane, fuse with the lateral nasal and the globular 

 processes, so completing the lateral parts of the primitive cranial lip (Figs. 64, 65, 66). 



The nodular outgrowths which form the rudiments of the auricles appear on the 

 margins of the hyo-mandibular cleft and fuse together, and by the end of the week traces 

 of the tragus, the helix, and the antitragus are visible (Fig. 103). 



By the seventh week the embryo has attained a length of 17 mm. (CR). The cervical 

 flexure has begun to unfold. The rudiments of the eyelids have appeared. The globular 

 processes have fused together, but there is still a distinct notch in the middle of the 

 cephalic or upper lip. The margins of the auricles are now well defined ; the hands are 



