288 



STUDY OF YOUNG CHICK EMBRYOS. 



nopleure, Spl, between the embryonic and extra-embryonic regions. The ento- 

 derm is a little thicker in the embryo than in the extra-embryonic territory. A 

 similar difference may be observed in the ectoderm. The embryonic mesoderm in 

 both somatopleure and splanchnopleure is considerably more developed and much 

 denser than in the extra-embryonic parts. The axial structures of the embryo — 

 namely, the spinal cord, Sp. c, and notochord, nch — are about the same as further 

 forward, but the mesoderm is much less advanced than further headwards, as is 

 evidenced by the small amount of mesenchyma above the axial structures and 

 by the slight differentiation of the mesothelium. The condition of the segments 

 and their relations to the somatic and splanchnic mesoderm are closely similar to 

 those represented in figure 32. Each segment consists of a larger part, Seg, of 

 rounded outline, close to the medullary tube, and of a narrower part, the nephro- 



FiG. 163.— Section of a Chicken Embryo with Twenty-eight Segments. Transverse Series 92, 



Section 419. 

 Cce, Ccelom. Cce', Diverticulum of the coelom. E>it, Entoderm. In, Intestinal cavity, nies, Mesoderm, nc/i, 



Notochord. Som, ScJmatopleure. Sp.c, Spinal cord. Spl, Splanchnopleure. S.z, Segmental zone. X 5° 



diams. 



tome, A^, which connects the inner portion of the segment with the lateral meso- 

 derm. The secondary segment consists of a distinctly marked wall which 

 extends around underneath the ectoderm and against the side of the medullary 

 tube, and of a thick inferior wall which fills up also the center of the segment. 

 Between the nephrotome and the entoderm are small blood-vessels, Ve. 



Section through the Posterior Portion of the Open Intestine (Fig. 163). — This 

 section is similar to the last, but we may note especially the f olio wmg 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, 5. z, which constitutes the segmental 

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

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



