86 



THE EARLY DEVELOPMENT OF MAMMALS. 



a rounded form, and a cavity appears in its interior; it is then often called a 

 segmental vesicle (Fig. 47, Nephr}. The exact details of the process by which the 

 nephrotome is separated from the other parts of the middle germ-layer have not 

 yet been carefully studied. Each nephrotome is the anlage of one of the excretory 

 tubules of the Wolffian body. It elongates into a tubule, which takes an S-shape, 

 and extends in the transverse plane of the body. The lateral end of the tubule 

 unites with, and acquires an opening into, the Wolffian duct. The median end 

 expands and produces a nephric corpuscle, with the characteristic glomerulus and 

 capsule. 



Am. 



Som. 



Ao. 



Nch. 



FIG. 47. SECTION OF A VERY YOUNG CAT EMBRYO. TRANSVERSE SERIES 413, SECTION 181. 

 Am, Amnion. A o, Aorta. Md, Medullary tube (spinal cord). My, Outer, My', inner wall of primitive segment 

 Nch, Notochord. Nephr, Nephrotome (segmental vesicle). Som, Somatopleure. Spl, Splanchnopleure. 

 Ve, Blood-vessel. W.D, Wolffian duct. X 50 diams. 



The portion of the somite which is isolated by the formation of the nephro- 

 tome lies, of course, next to the medullary canal. The term primitive segment (as 

 also proto-vertebra) is often applied to this structure as well as to the original 

 somite before the separation of the nephrotome, but it would be better to refer to 

 it as the secondary somite* The secondary somite, when first formed, appears more 

 or less nearly square in surface views, and triangular in cross-sections. As the 

 medullary canal grows, so does the secondary somite, and it becomes, therefore, 

 somewhat elongated in its dorso-ventral diameter. After this change in its shape 

 we can distinguish in transverse sections of an embryo (Fig. 38) the outer wall, 

 which lies under the ectoderm, and an inner wall, which lies toward the medullary 

 canal and notochord. In the further history of the somite we can distinguish the 

 following steps: first, the production of the dermatome (cutis plate) with the ac- 

 companying transformation of a portion of the cells of the inner wall of the -seg- 

 ment into the mesenchyma; next, the production of the true muscle-plate;; thirdly, 



* This is a new term, here proposed for the first time. 



