198 



STUDY OF YOUNG CHICK EMBRYOS. 



are the anlages of the omphalo-mesaraic veins, which gradually grow out and 

 branch in the extra-embryonic region, enlarging at the same time (compare Fig. 

 53). The general form of the embryo is indicated by figure i$>. In the region 

 of the head we notice the very well-marked head-bend which is established in the 

 region of the mid-brain, M.b. The medullary tube in the region of the head is 



M.b. 



A.p. 



Am.f. 



A.o. 



bl.is. 



Seg.z. J 



Aid. 



Pr.g. 



FIG. 150. EMBRYO CHICK WITH ABOUT TWENTY-FOUR SEGMENTS. SURFACE VIEW FROM THE DORSAL SIDE. 



A.c.Vf Amnio-cardiac vesicle. Am.f, Posterior edge of the amniotic fold. A.o, Area opaca. A.p, Area pellucida. 

 A.w, Arteria vitellina (or omphalo-mesaraica). bl.is, Blood-island in the area pellucida. Ht, Heart. M.b, 

 Mid-brain. Md, Medullary canal or spinal cord. Op, Optic vesicle. Ot, Otocyst. Pr.g, Primitive groove. 

 Seg, Primitive segment. Seg.z, Segmental zone, i, 2, Gill-arches. X 15 diams. 



very much enlarged and is divided into three well-marked primary cerebral vesicles, 

 which appear distinctly in specimens that have been stained and cleared. The 

 first of these is quite large, and at its side lies the anlage of the eye, Op, in the 

 center of which one readily distinguishes the commencement of the lens as a smalt 

 invagination of the ectoderm with its orifice still open. The second cerebral vesicle 

 is much smaller .than the first in every dimension. It occupies the region of the 

 head-bend and is separated from the first vesicle by a constriction, and from the 



