FIRST MONTH 



83 



slit-like external branchial pouches. The heart is a very prominent structure lying 

 immediately below the branchial region. About thirty-five pairs of segments can 

 be counted, and the tail forms a prominent rounded projection, from the under 

 aspect of which a short thick abdominal stalk connects the embryo with the 

 chorion, and contains the allantoic diverticulum and allantoic vessels. The 



hind-brain 



mid-brain 



amnion 



chorion 



abdom. stalk vilelline vein mouth fore-brain 



FIG. 113. EMBRYO OF 2'4 MM. ABOUT FOURTEEN DAYS OLD. (Kollmann.) 



stomodosum is a recess between the under aspect of the fore-brain and pericardial 

 region, and is separated by the buccopharyngeal membrane from the fore-gut. 



Coste's embryo is of about the same age as the one just described. 



By the end of the third week a stage is reached represented by His' embryo 

 lettered Lr (fig. 115). The embryo is almost completely cut off from the 

 yolk-sac, the mouth of which is narrowed into the yolk-stalk. The embryo 

 measures in a straight line between its most projecting anterior and posterior 

 ends 4'2 mm. The head end 

 is bent on the trunk in the 

 neck region, forming the cer- 

 vical flexure. The dorsi- 

 flexion of the body is no 

 longer seen, and the tail is 

 bent in towards the ventral 

 aspect. The abdominal stalk, 

 which is now lengthened out 

 somewhat, passes back to the 

 chorion on the right side of 

 the tail. 



The olfactory pit is begin- 

 ning to show on each side of 

 the fore-brain ; the optic 

 vesicles are prominent swell- 

 ings, but the lens pit is not 

 yet developed ; the auditory 



pits are closed, and the branchial region, triangular in shape, shows four branchial 

 depressions. The ridges bounding the branchial clefts are now named the 

 maiidibular, the hyoid, and the (three) branchial arches. The mandibular arch* 

 bounds the stomodceum behind, the hyoid lies between the first and second 

 branchial clefts, the branchial arches bound the remaining fissures. The hyoid 

 arch is the largest of the series. The heart, which now lies rather farther back 

 than before, shows distinct constrictions separating off its several parts. The 



o2 



yolk sac 



FIG. 114. EMBRYO OF 2'15 MM. (LETTERED Lg) ABOUT 



FIFTEEN DAYS OLD. (His.) 



