320 STUDY OF PIG EMBRYOS. 



their interior the cartilaginous anlages, cart', cart", of the skeleton of the limb, 

 and, around these, darker masses of tissue, the developing muscle-fibers. At the 

 lower edge of each limb is a blood-vessel, V.p, the so-called border or peripheral 

 vein, which extends completely around the edge of the developing hand and foot. 

 When the digits are developed, this vein becomes broken up, and out of its divi- 

 sions are formed the digital vessels. The section also passes through the penis, 

 Pen, in the center of which is the urethra, Ur. It shows here as a narrow 

 epithelial band without any cavity, except a very small one at its external 

 dorsal end. The band is lighter in the center, owing to the fact that the nuclei 

 are grouped chiefly close to the two surfaces of the band. At the base of the limb 

 is situated the irregularly shaped section of the iliac vein, V ' .11. In the median 

 line may be noted the following structures. Immediately underneath the nervous 

 system is the arteria sulci, A.sul. The vertebra, Vert, and notochord, Nch, resem- 

 ble corresponding structures in the section described on page 315, except that their 

 cytomorphosis is slightly less advanced. Below the vertebra lie the paired anlages of 

 the sympathetic nervous system, Sym, between which is the small median caudal 

 artery. The intestine, Reel, has its transverse diameter somewhat increased, so that 

 it appears oval in the section. Around it is beginning the differentiation of the 

 mucosa and muscularis. 



Section through the Mammary Anlage (Fig. 217). The figure 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 

 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 projects 

 somewhat outward and still more inward into the mesoderm. The epitrichium 

 passes continuously over the thickening, in the production of which it takes no 

 share. The inner edge of the ectoderm is marked by a very distinct line or base- 

 ment membrane, b, against the underlying mesoderm. The cells of the anlage 

 form two groups, one a band next to the basement membrane, in which the cells 

 present a somewhat radial arrangement, and the other a central group of cells, many 

 of which are elongated in a direction somewhat parallel to the surface of the 

 anlage, so that they form curving lines. The elongated cells in later stages gradu- 

 ally cornify and fall out, so that the anlage becomes hollow, but its excavation 

 proceeds very slowly, and in man is not usually completed until after birth. Soon 

 after the hollowing out of the anlage has begun, it sends out a series of buds from 

 its inner surface. These buds become elongated, somewhat twisted cords of cells, 

 and offer at this stage resemblance to embryonic sweat-glands. The outgrowths 

 subsequently branch and develop central cavities, and are ultimately transformed 

 into 'the secretory portion of the gland. 



Figure 217 also illustrates some important points in regard to the differentia- 



