STUDY OF SECTIONS OF EMBRYO OF 20 MM. 



321 



tion of the somatopleure. Parallel to the ectoderm, and some distance from it, is 

 a layer, Pan, which is marked out by numerous blood-vessels. This is the pan- 

 choroid layer. There is a slight but unmistakable difference in the mesoderm within 

 and without this layer, for in the region between the panchoroid and the ectoderm 

 the cells are somewhat more crowded. They probably represent the anlage of 



Mam. 



b. 



FIG. 217. PIG, 20. o MM. TRANSVERSE SERIES 59, SECTION 1043. 



b, Basement membrane of epidermis, -cost, Costal anlage. Cu, Cutis. EC, Ectoderm. Eptr, Epitrichium, 

 Mam, Mammary anlage. tnes, Mesenchyma. msth, Mesothelium. Pan, Tunica panchoroidea. per. in. 

 Peritoneal mesoderm. v e, Blood-vessel. X 250 diams. 



the cutis, Cu, and of the cuds only. Within the vascular layer the mesodermic 

 cells, mes, are not so near to one another, and the processes, by which they are 

 connected, are more slender. Toward the mesothelium is a broad band of denser 

 tissue, cost, the rudiment of a rib, the inner boundary of which is further marked 

 by several blood-vessels, ve. Between the costal anlage and the mesothelium is a 

 layer of embryonic connective tissue, the cells of which are more crowded toward 



