STUDY OF SECTIONS OF EMBRYO OF J 7 MM. 



more, regularly shaped nuclei. In the center of the vertebra lies the round noto- 

 chord, Nch, the sheath of which has increased considerably in thickness, and, 

 being unstained, appears as a clear space between the cells of the notochord 

 and those of the enclosing vertebra. The nuclei in the notochord are numerous 

 and somewhat crowded together. (2) The costal processes, Cost, of the vertebra, 

 which are rod-like and extend quite far down into the somatopleure. The histo- 

 genetic changes in these processes are similar to those in the vertebra, but less 

 advanced. They have progressed somewhat more in the proximal than in the 

 distal portion of the rib. (3) Around the central nervous system the pia mater 

 has become more distinct, and the arachnoid membrane is indicated by the wide 

 separation of its cells and the length of the processes connecting them. Its dif- 

 ferentiation is most easily recognized at the sides of the spinal cord. The outer 

 limit of the arachnoid is shown by a slight condensation of the mesenchyma which 

 marks the first step in the differentiation of the dura mater, the anlage of which 

 is further defined by the elongated form of the mesenchymal cells, by which they 

 differ from the mesenchymal cells on both sides. (4) There is a distinct layer of 

 condensed mesenchyma around the aorta, Ao. The layer thus formed consists of 

 elongated cells, and perhaps corresponds only to the muscular coat of the vessel. 

 (5) About the oesophagus, (E, the mesenchyma forms two distinct layers. The 

 inner, next to the epithelium, is of looser texture, and is the anlage of both the 

 mucous and submucous layers of the adult. The outer layer is denser and con- 

 sists chiefly of young smooth muscle-cells, which are merely modified mesen- 

 chymal cells, characterized by the greater development of their protoplasm and 

 by their elongated form. Traces of the differentiation of the outer layer into 

 the inner circular muscular coat and the outer longitudinal coat of the adult are 

 clear iru the section. 



spinal cord, Sp.c, has changed its outline as seen in section, being 

 brc^olest in the ventral zones, which have also begun to expand ventralward 

 so Ahat the outline of the cord shows on its ventral side a concavity, the first 

 idication of the ventral fissure. The three layers of the spinal cord are 

 \Tery distinct. The change in form, however, it can be clearly seen, is due chiefly 

 to the growth of the gray layer, tin, especially in the ventral zone. The gray 

 layer in the dorsal zone is still very slightly developed. From the dorsal zone 

 descends on either side the dorsal nerve-root, D.R, which presently joins 

 the ganglion, G. The ganglion now occupies a much lower position than in the 

 earlier stages (compare Fig. 198, G). From the ventral zone springs the ventral 

 root which unites with the dorsal at the lower tip of the ganglion. From the 

 nerve-trunk thus formed there is given off almost immediately the dorsal branch, 

 R.D, which soon ramifies in the midst of a dark mass of tissue, the anlage of the 

 dorsal musculature, muse. The main nerve-trunk descends ventralward and sends 

 off at the level of the vertebra a sympathetic branch, R.sy, which runs obliquely 

 downward and inward toward the aorta, and there terminates in the anlage of the 



