290 



STUDY OF YOUNG CHICK EMBRYOS. 



with the wall of the caudal intestine, cau. i. If the sections just in front are 

 studied carefully, it can be easily observed that the notochord also passes 

 over without boundary into the same band of cells, which is a mass repre- 

 senting the fusion of the walls of the medullary canal of the intestine and of the 

 tissue of the notochord. In this fused tissue we can, with our present means, 

 detect no signs of the coming differentiation. Just as the walls of the caudal 

 intestine are fused with the tissues on the dorsal side, so also are they fused on the 

 ventral side with the tissue of the allantois. If we follow the tissues laterally, 

 we see that they merge into the mesoderm proper. From the mesoderm there 

 has been a distinct upgrowth of tissue of rather loose texture on either side of the 

 medullary canal to form the segmental zone, 5. 2. 



Section through the Allantois behind the Intestine (Fig. 165). — This section is 

 only three in the series beyond that last described, yet it is posterior to the caudal 



£c. Mes. 



Sp.c. nch. S z. 



Som. 



mes/ Ent. All. Ve. mslJi. Spl. 



Fig. 165. — Chick Embryo with about Twenty-eight Segments. Transverse Series 92, Section 427. 

 All, Allantois. Cce, Coelom. Ec, Ectoderm. Ent, Entoderm. I\Ies, Mesoderm, vies' , Splanchnic mesoderm. 



msth, Mesothelium. nch, Notochord. Sotn, Somatopleure. Sp.c, Spinal cord. Spl, Splanchnopleure. S.z, 



Segmental zone of mesoderm. Ve, Blood-vessel. X 5° diams. 



intestine and shows, therefore, more completely the fusion of the structures in 

 the axial region. Except for the absence of the caudal intestine, the description 

 of the last section might apply also to this. The shape of the spinal cord, Sp. c, 

 is somewhat different, and its merging on the ventral side with the underlying 

 tissues is more marked. The cavity of the allantois is smaller and almost slit- 

 like. The other differences do not call for special description. 



Horizontal Section. — The student will find it profitable to make a series of 

 sections in the horizontal plane, trying to cut them as nearly as possible parallel 

 with the median plane of the fore-brain and mid-brain. 



The accompanying figure i66 is from a section of such a series. It shows 

 very clearly the general form of the embryo, the curvature of the neck, the sharp 

 angle of the head-bend, and the almost straight body. In the section repre- 

 sented the long stretch of the cavity of the fourth ventricle or hind-brain, Ven. IV, 



