490 THE HUMAN EMBRYO. 



At each side of the neck there are a couple of slit-like de- 

 pressions, HM, and HC.I, transverse to the axis of the embryo. 

 These are the external grooves, which lie opposite the hyoman- 

 dibular and first branchial gill-pouches, or diverticula from the 

 pharynx. The epiblastic grooves and the corresponding hypo- 

 blastic pharyngeal pouches lie in close contact with one another, 

 but do not communicate, so that there are at this stage no com- 

 plete gill-clefts, or actual perforations in the wall of the neck. 



The hyoid arch is the ridge or strip of the neck lying 

 between the first branchial and hyomandibular grooves, HC.I and 

 HM. The mandibular arch, MX, forms a much more prominent 

 ridge, in front of the hyomandibular groove ; and, wedged in 

 between the dorsal end of the mandibular arch and the under 

 surface of the head, is the comparatively small maxillary arch. 



The stomatodseum, or mouth depression, is a shallow pit on 

 the under surface of the head, bounded in front by the head 

 itself, at the sides by the maxillary arches, and behind by the 

 mandibular arches. It does not yet open into the fore-gut 

 (cf. Fig. 232). 



The heart, R, is large, and lies immediately below the bran- 

 chial region of the neck, between this and the yolk-sac ; it is a 

 single tube, twisted so as to form a strongly curved loop, with 

 its convexity towards the right side of the embryo. 



In the body region, the outlines of the somites or proto- 

 vertebrse can be seen through the skin ; about thirty-five pairs 

 being already present. The tail, TL, forms a prominent rounded 

 stump ; and from the under surface of its base the short thick 

 allantoic stalk, TZ, arises, which attaches the embryo to the 

 chorion. There is as yet no trace of the limbs. 



An embryo lettered by Professor His, Lr (Figs. 195 and 198), 

 and estimated as twenty or twenty-one days old, may be taken 

 as typical of the condition attained by the end of the third 

 week. 



Apart from the great increase in size, best seen by comparing 

 Figs. 189 and 195, p. 487, the chief points in which the embryo 

 Lr differs from the embryo Lg, are : The narrowing of the yolk- 

 stalk, by which the embryo is separated more markedly from 

 the yolk-sac ; and the almost complete disappearance of the 

 sharp bend in the back, opposite the yolk-sac, which is so 



