196 J. A. MYERS 



mary streak is present on each side. Each streak consists of a 

 single layer of cubical epithelium. The breadth of the streak 

 has increased and now extends from a few cells dorsal to the 

 dorsal limiting furrow ventrally to cover nearly half of the 

 parietal zone. Its cephalic and caudal ends blend with the 

 cubical epithelium of the limb anlages. 



At twelve days and thirteen hours, the cells of the mammary 

 streak are larger and in the region of the dorsal limiting furrow a 

 second layer of cells is beginning to appear superficial to the 

 cubical cells. Immediately beneath the mammary streak the 

 mesenchymal cells have condensed. The mammary streak 

 shows two distinct cell layers in embryos of thirteen days and 

 one hour. The superficial layer — stratum corneum — consists of 

 flat cells with oval nuclei with their long axes parallel to the 

 surface. The deep layer — stratum mucosum — is composed of 

 large round or cubical to cylindrical cells with oblong nuclei. 

 The streak is separated from the mesenchyma by a distinct light 

 line-^the basement membrane. 



Henneberg found the first appearance of the mammary line 

 in a rat embryo of thirteen days and fourteen hours. At this 

 stage it is produced by a thickening of parts of the mammary 

 streak. In some places a part of the mammary streak is con- 

 verted into the mammary line by the appearance of a third layer 

 of round cells between the superficial and deep layers. In other 

 places the cells have slightly thickened thus producing the first 

 appearance of the mammary line without the addition of a third 

 layer. In other embryos of the same age the mammary line 

 in the thoracic region is three to four layers of cells thick and its 

 greatest breadth shows twelve to fourteen layers of cells. It dis- 

 appears a short distance cephalad to the anterior extremity. 

 In the inguinal region the line is still very indistinct and requires 

 special technique for its study. In some embryos a complete 

 interruption exists between the region of the future thoracic 

 glands and the • adbominal gland. This is the first intimation 

 of the future interspace between the glands of the thoracic 

 region and those of the abdominal and inguinal regions. From 

 this stage, Henneberg designated the cephalic part of the line 



