EMBRYO OF 17 MM. 233 



groups of mesothelial muscle cells, the anlages of the various skeletal muscles. 

 There is little differentiation otherwise in the mesenchyma, but we may note the 

 following changes in it : (1 ) The anlage of the vertebra, Vert, which is now quite 

 well defined ; around the edge of it the cells have assumed an elongated form and 

 have elongated nuclei; the elongation is parallel with the surface of the anlage. 

 These cells result from the commencing differentiation of the perichondrium, 

 which at this stage merges on the one side into the anlage of the vertebrae, and on 

 the other into the surrounding mesenchyma. The cells of the vertebra have 

 changed into young cartilage cells. They are now distinctly separated from one 

 another by a well-developed matrix. Each cell occupies a separate space or 

 capsule in the matrix. The protoplasm of the cell having almost completely 

 disappeared, only the nucleus remains distinct. It stains readily, has a distinct 

 outline, and contains a number of dark granules, one or two of which are conspicu- 

 ous by their greater size and irregular shape. The nucleus itself, in most of the 

 cells, is somewhat irregular in outline, as if distorted by shrinkage. Toward the 

 center of the anlage the cytomorphosis is most advanced. Toward its outer sur- 

 face the cells are less changed, lie nearer together, and have more regularly 

 shaped nuclei. In the center of the vertebra lies the round notochord, 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 en- 

 closing 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 histogenetic 

 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 differentiation 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, CE, 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- 



