HUMAN EMBRYO IN THE ELEVENTH STAGE. 



147 



Human Embryo in the Eleventh Stage with the Cervical Sinus in Formation. 



The embryo figured (Fig. 95) was described by Mall, and one almost identical 

 has been studied by H. Piper. Its age is probably twenty-six days. At this 

 stage the embryo is flexed so as to describe almost a circle, the tail being almost 

 in contact with the head, yet comparison with figure 94 reveals that the straightening 

 of the back of the embryo has begun. Although the limbs, A.I and P.I, have 

 increased in size, they are still only rounded buds. The head, which is bent to 

 the right, partly conceals the cardiac region. The nasal pit,, Na, is a broad, 



IV 



al 



s.s 



FIG. 94. HUMAN EMBRYO OF ABOUT TWENTY- 

 THREE DAYS, 4 .o MM. X 15 diams. (After 

 W. His, Embryo a.) 



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

 Optic vesicle, pi, Posterior limb-bud, iv, 

 Fourth ventricle, i, Mandibular process. 

 2, Hyoid arch. 3, 4, Third and fourth gill- 

 arches. 



Cerv.s 



A.I. 



P.I. 



FIG. 95. HUMAN EMBRYO OF 7 .o MM. X 8 diams. 



(After F. P. Mall.) 



A.I, Anterior limb. Cerv.s, Cervical sinus. Li, Liver. 

 Md, Mandibular process. MX, Maxillary process. 

 Na, Nasal pit. Op, Eye. P.I, Posterior limb. 

 Um.c, Umbilical cord. Yk.s, Yolk-stalk. 



shallow fossa. The eye, Op, consists of the small optic vesicle and overlying lens. 

 The maxillary process, MX, is well developed. Behind the mandibular process, 

 Md, is the first cleft, or anlage of the external auditory meatus. The cervical 

 sinus, Cerv.s, is in process of development, but on the left side is not so deep as 

 on the right side, which is figured. The ventral ends of the branchial arches are 

 continuous with the cardiac region of the body. As shown in the figure, twenty- 

 four segments are clearly marked externally. The large cardiac region fills out 

 the space between the anterior 'limb, A.I, and the tip of the head. The ventral sur- 

 face of the abdomen is prolonged to form the umbilical cord, Um.c, from which 

 projects the slender yolk-stalk, Yk.s. The position of the liver is indicated by a 

 distinct protuberance below the foreleg. 



