HUMAN EMBRYO OF TWENTY-SIX DAYS. 



143 



the arm. The position of the optic vesicles can be clearly recognized by a small 

 external protuberance. The four external gill clefts still show clearly. Between 

 the first gill cleft and the mouth lies the mandibular process, i ; between the first 

 cleft and the second lies the hyoid or second branchial arch. The third and 

 fourth branchial arches are also distinct, but the fifth branchial arch, or that be- 

 hind the fourth gill cleft, no longer appears externally, but has been turned 

 inward, this turning-in marking the first step toward the development of the 

 cervical sinus. The tail overlaps the pericardial chamber. The body-stalk, 

 B. S., comes off on the left side of the 

 embryo, so that when the specimen was 

 obtained it lay with its left side against 

 the chorion, the body-stalk being quite 

 short. The specimen was derived from 

 a chorionic vesicle measuring 25 X 30 

 mm. The greatest length of the embryo 

 was 4 mm. 



Human Embryo of Twenty=six Days. 



The embryo figured (Fig. 80) was de- 

 scribed by Mall. Another specimen almost 

 identical has been studied by H. Piper. 

 At this stage (Fig. 80) the embryo is flexed 

 upon itself, forming almost a complete 

 circle. The limb-buds have increased in 

 size. The posterior branchial arches are 

 in process of disappearance by invagi- 

 nation on the ectodermal side to form 

 the cervical sinus. The three branchial 

 arches are visible on the right side, as 

 shown in the figure, but four can still be 

 seen on the left. The body-stalk is now 



merged in the formation of the true umbilical cord (compare page 109). The head 

 is bent down so as partially to overlap the pericardial region. On its ventral 

 side it shows a large, shallow depression, Na, which is the anlage of the future 

 nasal cavity. The eye is marked by the commencing formation of the lens, 5. 

 The mandibular process, Md, represents the lower boundary of the mouth, but it 

 has an extension toward the high nasal pit. This extension, Mx, is known as the 

 maxillary process. The first and second gill clefts, i and 2, can be easily seen, 

 and that which might at first be mistaken for the third, Cerv. s, is really the 



Fig. 81. — Human embryo 7.5 mm. in Maximum 

 Length. — {After W. His.) 



