In the earliest of these sections there is not a trace of a folding off of the embryo from the yolk, and the body walls are quite horizontal. In the second section (fig. 116), from an embryo of about two days, the body walls are already partially inclined, and the splanchnopleure is very distinctly folded inwards. There is a considerable space between the notochord and the hypoblast, which forms the rudiment of the mesentery. In the third section (fig. 117) the body walls have become nearly vertical, the folding of the splanchnopleure is nearly completed, and it is only for a small region that the alimentary tract is open, by the vitelline duct, to the yolk-sack. [FIG. 118. EMBRYO CHICK AT THE END OF THE FOURTH DAY SEEN AS A TRANSPARENT OBJECT. The amnion has been completely removed, the cut end of the somatic stalk is shewn at _S.S._ with the allantois (_Al_) protruding from it. _C.H._ cerebral hemisphere; _F.B._ vesicle of the third ventricle with the pineal gland (_Pn_) projecting from its summit; _M.B._ mid-brain; _Cb._ cerebellum. _IV. V._ fourth ventricle; _L._ lens; _ch.s._ choroid slit. Owing to the growth of the optic cup the two layers of which it is composed cannot any longer be seen from the surface, but the retinal surface of the layer alone is visible. _Cen. V._ auditory vesicle; _s.m._ superior maxillary process; _1 F_, _2 F_, etc. first, second, third and fourth visceral arches; _V._ fifth nerve sending one branch to the eye, the ophthalmic branch, and another to the first visceral arch; _VII._ seventh nerve passing to the second visceral arch; _G.Ph._ glossopharyngeal nerve passing towards the third visceral arch; _Pg._ pneumogastric nerve passing towards the fourth visceral arch; _iv._ investing mass. No attempt has been made in the figure to indicate the position of the dorsal wall of the throat, which cannot be easily made out in the living embryo; _ch._ notochord. The front end of this cannot be seen in the living embryo. It does not end however as shewn in the figure, but takes a sudden bend downwards and then terminates in a point. _Ht._ heart seen through the walls of the chest; _M.P._ muscle-plates. _W._ wing; _H.L._ hind limb. Beneath the hind limb is seen the curved tail.] These three sections further illustrate (1) the gradual differentiation of the mesoblastic somites (fig. 115, _P.v_) into (_a_) the muscle-plates (figs. 116, _ms_ and 117, _m.p_), and (_b_) the tissue to form the vertebral bodies and adjacent connective tissue; (2) the formation of a mass of tissue between the lateral plates and the mesoblastic somites (fig. 115), known as the intermediate cell mass, on the dorsal side of which the Wolffian duct is formed, while the intermediate cell mass itself breaks up into the segmental tubes (fig. 116, _st_) and connective tissue of the Wolffian body. [FIG. 119. SECTION THROUGH THE LUMBAR REGION OF AN EMBRYO CHICK AT THE END OF THE FOURTH DAY.