_n.c._ neural canal; _p.r._ posterior root of spinal nerve with ganglion; _a.r._ anterior root of spinal nerve; _A.G.C._ anterior grey column of spinal cord; _A.W.C._ anterior white column of spinal cord just commencing to be formed, and not very distinctly marked in the figure; _m.p._ muscle-plate; _ch._ notochord; _W.R._ Wolffian ridge; _AO._ dorsal aorta; _V.c.a._ posterior cardinal vein; _W.d._ Wolffian duct; _W.b._ Wolffian body, consisting of tubules and Malpighian bodies; _g.e._ germinal epithelium; _d._ alimentary canal; _M._ commencing mesentery; _SO._ somatopleure; _SP._ splanchnopleure; _V._ blood-vessels; _pp._ pleuroperitoneal cavity.] Various other features in the development of the vascular system, general mesoblast, etc., are also represented in these sections. It may more especially be noted that there are at first two widely separated dorsal aortæ, which gradually approach (figs. 115 and 116); and meeting first of all in front finally coalesce (figs. 117 and 119) for their whole length. The general appearance of the embryo of the fourth day may be gathered from fig. 118. [FIG. 120. HEAD OF A CHICK FROM BELOW ON THE SIXTH AND SEVENTH DAYS OF INCUBATION. (From Huxley.) _I^a._ cerebral vesicles; _a._ eye, in which the remains of the choroid slit can still be seen in A; _g._ nasal pits; _k._ frontonasal process; _l._ superior maxillary process; _1._ inferior maxillary process or first visceral arch; _2._ second visceral arch; _x._ first visceral cleft. In A the cavity of the mouth is seen enclosed by the frontonasal process, the superior maxillary processes and the first pair of visceral arches. At the back of it is seen the opening leading into the throat. The nasal grooves leading from the nasal pits to the mouth are already closed over and converted into canals. In B the external opening of the mouth has become much constricted, but it is still enclosed by the frontonasal process and superior maxillary processes above, and by the inferior maxillary processes (first pair of visceral arches) below. The superior maxillary processes have united with the frontonasal process, along nearly the whole length of the latter.] The changes which have taken place consist for the most part in the further development of the parts already present, and do not need to be specified in detail. The most important event of the day is perhaps the formation of the limbs. They appear as outgrowths from a slightly marked lateral ridge (fig. 119, _WR_), which runs on the level of the lower end of the muscle-plates for nearly the whole length of the trunk. This ridge is known as the Wolffian ridge. The first trace of the limbs can be seen towards the end of the third day; and their appearance at the end of the fourth day is shown in fig. 118, _W_ and _HL_.