224 STUDY OF PIG EMBRYOS. 



proper, which occurs at the level of the mid-brain, the nature and significance of 

 which become clearer when the disposition of the nervous system is studied (compare 

 Fig. 178). From the mid-brain one axis extends backward through the region of 

 the hind-brain, Ven, to the neck-bend; the other axis extends vertically downward 

 to the region of the fore-brain. On the surface of the head we find the nasal pit, 

 AT", distinctly marked. The eye, Op, shows clearly the outlines of the optic vesicle 

 and of the lens in the center. It is entirely without lids. The small size of the 

 eye is a characteristic of the mammalian embryo by which it differs from all saurop- 

 sidian forms; but, as previously stated, the embryonic eye is slightly larger in certain 

 other mammals. Below the eye is the maxillary process, MX, which is destined to 

 form 'the greater part of the upper jaw. The anterior boundary of the maxillary 

 process is marked, as before, by the lachrymal groove, which runs now from the 

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

 which the lower jaw is to be developed, is separated from the maxillary process by 

 a groove, the boundary between the upper and lower jaws, and is bounded . 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 cavity of the hind-brain is very large and is known as the fourth ventricle, 

 Ven; as it has a very thin roof it can be readily distinguished. The thickened 

 floor of the fourth ventricle is the anlage of the medulla oblongata. The opening 

 of the cervical sinus, C.S., is triangular, as before; within it are hidden the third, 

 fourth, and fifth branchial arches. In slightly older embryos the orifice of the 

 sinus is further contracted, becoming a small rounded opening which finally closes 

 over completely. The territory of the mandibular process and cervical sinus corre- 

 sponds to the pharyngeal region. It is the site of some of the most important, 

 interesting, and complicated developments by which the embryonic is changed into 

 the adult anatomy. 



The dorsal outline of the body forms a long sweeping curve, ending in the 

 tail. Comparison with figure 165 shows at once that the straightening out of the 

 dorsal region is begun, yet at this stage the dorsal side of the embryo is nearly 

 three times as long as the ventral. The umbilical cord has grown in length, and 

 is constricted in diameter as it joins the abdomen, yet its connection with the body 

 occupies practically the entire length of the ventral median line. The position and 

 number of the segments, Seg, is still shown by the external modeling. Both limbs 

 are well advanced, the anterior, A.L., more so than the posterior. From the base 

 of the brain to the base of the hind limb extends the milk-line, M.L, curving so as 

 to be nearly parallel, to the dorsal outline of the body. Along it the mammary glands 

 are ultimately developed. Extending across the body are several shadowy lines 

 shimmering through the translucent body-walls. One marks the position of the 

 embryonic diaphragm; it extends from the upper edge of the anterior limb ob- 

 liquely downward toward the edge of the umbilical cord. Another, which extends 



