EXTERNAL FORM OF EMBRYO OF lo MM. 161 



less developed in birds and reptiles, and being absent in amphibians and fishes. 

 It is probably closely correlated with the cramping of the ventral cervical region, 

 which leads to the formation of the cervical sinus, C.S, and to the disappearance 

 of the second to fourth gill clefts. The neck bend is so great that the mandibular 

 and nasal regions of the embryo are closely appressed to the cardiac region of the 

 body proper. The cephalic region has a second flexure, the head bend proper, 

 which marks the site of the mid-brain, and in the figure appears as a rounded 

 angle obliquely above the eye. From the mid-brain one axis, horizontal in the 

 figure, extends backward through the region of the fourth ventricle, or hind- 

 brain. Yen, to the neck bend, while the other axis extends vertically downward 

 to the region of the fore-brain, which is marked by a rounded protuberance in the 

 outline of the head. The dorsal outline of the body proper forms a long sweeping 

 curve, ending in the tail; this dorsal curvature is another characteristic of the 

 amniote embryo, the back in the embryo of fish-like forms being relatively 

 straight. It is thus brought about that the dorsal side of the embryo is two or 

 three times as long as the ventral. From the ventral side springs the large um- 

 bilical cord, the connection of which with the body occupies practically the entire 

 length of the ventral median line of the abdominal region proper. Above the 

 umbilical cord the protuberant outline of the cardiac region passes below the 

 nasal, A^, and mandibular, Md, regions toward the cervical sinus, C.S. The 

 long tapering tail extends near the umbilical cord. 



The surface modeling of the embryo offers important features. Beginning 

 with the head, we observe first the shallow depression, constituting the nasal pit, 

 A^, The eye, Op, is entirely without lids; the lens appears in the center and is 

 surrounded by the outlines of the optic vesicle. The small size of the eye is a 

 characteristic of the mammalian embryo ; by which it differs from all saurop- 

 sidan forms, but in certain other mammals the embryonic eye is slightly larger 

 than in the pig. Below the eye is the maxillary process, M%, which is destined 

 to form the greater part of the upper jaw; the anterior boundary of the maxillary 

 process is marked by a shallow depression, the lachrymal groove, which runs 

 from the angle of the eye. Op, to the nasal pit, N . The mandibular process, 

 Md, out of which the lower jaw is developed, is bounded in front by a groove 

 separating it from the maxillary process, and behind by a second groove, Au, 

 the anlage of the future meatus auditorius externus. This groove marks the 

 boundary between the mandibular process and the first or hyoid branchial arch, 

 and is itself the ectodermal member of the first gill cleft. The fourth ventricle, 

 Yen, or cavity of the hind-brain, having very thin walls for its roof, can be read- 

 ily distinguished. The thickened floor of the fourth ventricle is the anlage of 

 the medulla oblongata. The cervical sinus, C.S, is an area of invagination, 



