EMBRYO OF 20 MM. 



249 



Section through the Mammary Anlage. Figure 144 represents a section 

 through the somatopleure of the embryo in the region of a mammary gland. The 

 ectoderm, EC, covers the external surface of the somatopleure, as does the meso- 

 thelium, msth, the inner surface, the space between the two covering layers being 



Pan. 



Eptr. 



- Mam. 



% r 





P1V5,- ' 'iiXWf 



FIG. 144. PIG, 20.0 MM. TRANSVERSE SERIES 59, SECTION 1043. 



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

 Mam, Mammary anlage. ines, Mesenchyma. msth, Mesothelium. Pan, Tunica panchoroidea. per. m, 

 Peritoneal mesoderm. ve, Blood-vessel. X 2 5 diams. 



occupied by various mesodermic structures. The ectoderm consists of two or 

 three layers of cells, the external one of which, Eptr, the epitrichium, is very thin. 

 To form the mammary anlage, Mam, the ectoderm suddenly thickens and pro- 

 jects somewhat upward and still more downward into the mesoderm. The epi- 



