; HUMAN EMBRYO AT DIFFERENT STAGES. 63 
area is no longer on the surface of the ovum, for the amnion folds have closed. The yolk 
sac is formed, and the rudiment of the allantoic duct projects backwards from its upper 
and posterior part. Mesoderm has formed, and it has extended round the yolk sac and 
over the inner surface of the chorion. The embryonic area, with the yolk-sac and the 
amnion, are enclosed within the blastoderm, but they remain attached to the inner surface 
of the chorion by a relatively thick stalk of ectodermal and mesodermal tissue, the body- 
stalk, which is subsequently replaced by the umbilical cord. The outer surface of the 
ovum, which now consists entirely of chorion, is covered with small villi into some of 
which mesodermal cores are projecting. 
Obviously the ovum of the latter part of the twelfth day differs considerably from 
that of the earlier part of the same day, but the transitional stages which intervene 
between the two have not yet been observed. Probably, however, the inner granular 
layer of cells in the embryonic area, which represent the entoderm, increase and form a 
solid mass in which a cavity soon appears. Directly after the formation of the cavity in 
the entoderm the primitive streak appears, and the mesoderm, growing from it rapidly, 
covers the entodermal sac and spreads over the inner ‘surface of the chorionic area. At 
the same time the amniotic folds form and separate from the chorion, but this separation is 
not effected till the mesoderm, extending backwards from the posterior end of the embryonic 
area, has reached and becomes connected with the inner surface of the chorion. Consequently, 
when the amniotic folds fuse together and separate from the remainder of the blastoderm, 
the embryonic area, with the yolk-sac and amnion, still remain attached to the inner 
surface of the chorion. 
During the thirteenth day the embryonic area is elevated, the cephalic and caudal 
folds are developed, and the pericardial region becomes prominent between the head 
extremity of the embryo and the upper and anterior part of the yolk-sac. The neural 
groove and the neural folds appear; the posterior ends of the folds embrace the anterior 
extremity of the primitive streak on which the primitive groove is formed. At the 
anterior end of the primitive groove a neurenteric canal appears, forming a communication 
between the neural groove and the posterior end of the primitive alimentary canal. 
On the fourteenth day the embryo is more distinctly separated from the yolk-sac ; the 
head increases considerably in size, and its anterior part is bent downwards. The posterior 
part of the neural canal is completed, except at the extreme end, by the meeting and fusion 
of the neural folds, but it is still open anteriorly, where traces of the cerebral vesicles are 
present. The two halves of the heart fuse together; the single tube thus formed is 
slightly bent upon itself, and its outline is visible from the exterior. The pericardial 
_ region increases in size, and a distinct stomatodzeal space appears between it and the 
anterior part of the head. The outlines of fourteen protovertebral somites are visible on 
the outer surface of the body. 
Fic. 49. 
D. Human embryo at the 21st day of development ; E. At the 23rd day of development ; 
F. At the 27th day of development. (After His.) 
AS. Allantoic stalk; BS. Body-stalk ; CV. Chorionic villi on a segment of the chorion ; EY. Eye; FL. Fore- 
3 limb; H. Head; HA. Hyoid arch; HL. Hind-limb; MA. Mandibular arch; MB. Mid-brain; MP. 
Maxillary process; OP. Olfactory pit ; OV. Otic vesicle ; PR. Pericardial region ; PS. Protovertebral 
somite ; SS. Stomatodwal space ; UC. Umbilical cord ; VC. Visceral cleft ; WR. Wolffian ridge; YS. 
Yolk-sac. 
The third week.—On the fifteenth day the auditory pits and two visceral clefts 
appear. The head and pericardial region enlarge, and the stomatodseal space, which 
‘increases simultaneously, becomes more defined laterally by the forward growth of the 
maxillary processes. 
