144 



THE HUMAN EMBRYO. 



opening of the cervical sinus, in which the gill clefts are partly buried. Between 

 the first andsecond gill cleftsappearsthesecondorhyoidbranchialarch. Twenty- 

 four segments are indicated externally on the right-hand side of the embryo 

 shown in the figure. Between the head and the anterior limb runs the large peri- 

 cardial region. Between the anterior limb, a. I, and the umbilical cord, Urn. c, 

 another protuberance shows externally, due to the development of the liver. 



Human Embryos of Twenty=seven to Twenty=eight Days. 



Embryos of this age are characterized by the extreme development of the 



neck bend (compare Fig. 83). As is well il- 

 lustrated in figure 81, the apex of the neck 

 bend forms, as it were, the summit of the em- 

 bryo, the greatest length of which is from 7 to 

 8 mm. As it changes from the twenty-sixth 

 day we note especially the increased distinct- 

 ness of the nasal pit and of the still open in- 

 vagination of the ectoderm to form the lens of 

 the eye, and, still more, the great deepening of 

 the cervical sinus. Although the cervical sinus 

 is an ectodermal structure, yet its formation is 

 due to modifications of the gill arches, the third, 

 fourth, and fifth of which become pushed in- 

 ward. At the stage we are considering, the 

 process of invagination, though far advanced, 

 is not completed. It results in the formation 

 of a deep fissure on each side of the neck, the 

 general nature of which may be understood 

 from the accompanying figure 82. The fifth 

 arch is turned in first, the fourth second, 

 and the third last. The sinus when fully 

 formed is very narrow, so that the surfaces of the branchial arches come in 

 contact with the opposite wall of the sinus. The ectoderm on two sides unites 

 and the sinus thus becomes closed, and is for a time represented by an epithelial 

 cord connected with the epidermis or superficial ectoderm. So far as known, 

 the cervical sinus entirely disappears, but its abnormal persistence may account 

 for certain cysts occurring pathologically in the neck. In all parts of the em- 

 bryo of twenty-eight days there is an obvious advance upon the conditions 

 found at twenty-six days. Comparison with one another of embryos in this 

 stage shows that there is considerable variation, especially as to the nature and 



*Ss3 





Fig. 82. — Reconstruction of the 

 Pharyngeal Region of a Human 

 Embryo of 11.5 mm. 



JV, Nasal pit. gl, Processus globularis. 

 hy. Hypophysis. Si, Sinus cervicalis. 

 Lit, Lung. MJ, Mandible. //, Sec- 

 ond branchial arch. X I0 diams. — 

 (After W. His.) 



