EMBRYO WITH TWENTY-EIGHT SEGMENTS. 



213 



section is similar to the last, but we may note especially the following differ- 

 ences: The spinal cord, Sp.c, shows a comparatively large cavity, which is widest 

 on the dorsal side, so as to be somewhat triangular in section. In place of the 

 segments we have only the mass of cells, S.z, which constitutes the segmental zone, 

 out of which later segments will be differentiated. The segmental zone, S.z, is 

 of a rather loose texture and merges without boundary into the somewhat denser 

 mesenchyma of the somatopleure and splanchnopleure of the embryo. The dense 

 tissue of the somatopleure extends much farther laterally than the corresponding 

 tissue in the splanchnopleure. The notochord, nch, is very large and fills out the 

 entire space between the ventral boundary of the spinal cord and the entoderm, 

 and though the mesoderm comes in contact with the notochord, it does not sur- 

 round it, the relations here representing an earlier stage of development than any 



Som. EC. Cce. cau.i. Sp.c. nch. S.z. 



Mes. 



Ent. 



All. 



FIG. 162. SECTION OF A CHICK EMBRYO WITH TWENTY-EIGHT SEGMENTS. TRANSVERSE SERIES 92, SECTION 424. 



All, Allantois. cau.i, Caudal intestine. Cce, Coelom. EC, Ectoderm. Ent, Entoderm. Mes, Mesoderm. 

 mes' ' , Splanchnic leaf of mesoderm. nch, Notochord. Som, Somatopleure. Sp.c, Spinal cord. Spl, Splanch- 

 nopleure. S.z, Segmental zone of mesoderm. Ve, Blood-vessel. X 50 diams. 



which we find further head ward. The entoderm, Ent, of the embryonic region is 

 considerably thickened and forms an intestinal channel, In, of very characteristic 

 form; for the top of this channel is nearly horizontal, while the sides are vertical 

 and form a distinct angle with the top. In the midst of the mesoderm, on either 

 side of the intestine, there is a small cavity, Cce', which in two or three sections 

 further forward is found to unite with the general cavity of the ccelom. The 

 morphological meaning of this special pocket of the body-cavity is unknown. 



From this point onward in the series changes in the appearance of the -sec- 

 tions take place very rapidly. The two sections next to be described are quite 

 close in the series to the present one. 



'Section through the Caudal Intestine (Fig. 162). In this section we encounter 

 the singular fusion of the germ-layers which is characteristic of the caudal extremity 

 of all vertebrate embryos during early stages. In the median line we see three 

 distinct cavities. The dorsal of these may be readily identified as the continuation 

 of the cavity of the spinal cord. The middle and ventral cavities are entodermal; 

 the upper of the two entodermal cavities, cau.i, represents a prolongation of 

 the entodermal cavity into the developing tail of the embryo (compare Fig. 16). 



