216 STUDY OF YOUNG CHICK EMBRYOS. 



well shown, and it can be readily seen that the hind-brain is nearly equal in 

 length to the mid- and fore-brains combined. In the floor of the hind-brain ap- 

 pears a series of curved notches corresponding to the neuromeres. Only a shaving 

 from the side of the mid-brain, M.b, and two similar shavings from the two parts 

 of the fore-brain, the diencephalon, Dien, and the cerebral hemispheres, H, appear 

 in the section. The optic nerve is cut across and appears as a hollow tube. 

 Underneath the hind-brain a piece of the pharynx, Ph, is cut, and below the 

 pharynx is the large projecting heart, which is very clearly shown to consist of 

 an inner or endothelial tube, Endo, and an outer mesothelial tube, m.ht, the anlage 

 of the muscular portion of the heart. The endothelial tube is cut twice; the 

 upper portion, Ao, is the aortic trunk, the lower portion, Ven, corresponding to 

 the ventricle. The heart is, as it were, suspended from the lower wall of the 

 pharynx. The entoderm of the pharynx is very thin on the dorsal side, and 

 thicker on the ventral side. Between the head and the pharynx one can see the 

 projecting mandibular process, Mdb. The small space to the right of this process 

 in the figure, between it and the head, corresponds to the cavity of the mouth. 

 Close to the mandibular process, on the side toward the heart, springs the amnion 

 of the embryo, Am, which passes close around the head of the embryo lying very 

 near it, and can be followed down to where it rejoins the posterior end of the 

 embryo, on the left-hand side of the figure. Underneath the posterior part of the 

 hind-brain can be seen a small piece of the notochord, nch. The notochord 

 appears twice more in the section, nch' and nch" , in the dorsal region of the em- 

 bryo. From the end of the hind-brain the cervical region curves to the right. 

 In it. there is a large cavity, D.Ao, the dorsal aorta. To the left of the dorsal 

 aorta we begin to get the primitive segments, which are very distinctly marked. 

 They become gradually wider and wider as we proceed toward the caudal end 

 of the embryo. There also they are less advanced in their development. A 

 small bit of the spinal cord appears in section, Md. From the extreme inferior 

 end of the section a prolongation of the somatopleure can be seen which also 

 leads off into the formation of the amnion, Am". There appears again a piece, 

 Sp.c, of the spinal cord and a fragment of the notochord, and on either side of 

 this a segmental zone, Seg.z, of the mesoderm. On the right there shows a small 

 portion of the body-cavity, Cos, distinctly bounded on, both sides. Its exterior 

 boundary is a piece of the true body-wall, Som, of the embryo, and close by it is 

 another portion of the amnion, Am'. How this is possible may be readily under- 

 stood by comparison of this figure with figure 161, which represents a transverse 

 section of a similar embryo in this region. 



Histological Differentiation of the Chick Embryo with Three Gill-clefts. 



It is important that the student make a thorough examination and study 

 with a high power of all the cells and tissues of the embryo at this stage so as to 

 familiarize himself with the embryonic characteristics of the germ-layers. The 



