EMBRYO WITH TWENTY-EIGHT SEGMENTS. 289 



mesenchyma of the somatopleure and splanchnopleure of the embryo. The 

 dense tissue of the somatopleure extends much further laterally than the corre- 

 sponding tissue in the splanchnopleure. The notochord 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 surround it, the relations here representing an earlier stage of development 

 than any which we find further head wards. The entoderm, Ent, of the embryonic 

 region is considerably thickened and forms an intestinal channel, In, of very char- 

 acteristic 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 coelom. 

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



Soin. EC. Cx. cau.i.. Sp.c. nek. S.z. Ales. 



Ent. All. 



FIG. 164. CHICK EMBRYO WITH ABOUT TWENTY-EIGHT SEGMENTS. TRANSVERSE SERIES 92, SECTION 424. 



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

 mes f , Splanchnic leaf of mesoderm. nek, Notochord. Som, Somatopleure. Sp.f, Spinal cord. Spl, 

 Splanchnopleure. S.z, Segmental zone of mesoderm. Ve, Blood-vessel. X 5 diams. 



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. 164). In this section we encounter 

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

 ity 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 continua- 

 tion of the cavity of the spinal cord. The middle and ventral cavities are ento- 

 dermal; the upper of the two entodermal cavities, can. i, represents a prolonga- 

 tion of the entodermal cavity into, the developing tail of the embryo (compare 

 Fig. 43). The lower cavity is the anlage of the allantois, All, which is destined 

 to grow out during the next few days into a relatively large round vesicle. The 

 tissue on the ventral side of the spinal cord, Sp. c, is connected by a band of cells 

 19 



