250 STUDY OF PIG EMBRYOS. 



trichium 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 basement 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 sur- 

 face of the anlage, so that they form curving lines. The elongated cells in later 

 stages gradually 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, some- 

 what twisted cords of cells, and offer at this stage resemblance to embryonic 

 sweat-glands. The outgrowths subsequently branch and develop central cavi- 

 ties, and are ultimately transformed into the secretory portion of the gland. 



Figure 144 also illustrates some important points in regard to the differen- 

 tiation 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 pan-choroid and the 

 ectoderm the cells are somewhat more crowded. They probably represent the 

 anlage of the cutis, Cu, and of the cutis only. Within the vascular layer the 

 mesodermic cells, mes, are less near to one another, and their 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 meso- 

 thelium is a layer of embryonic connective tissue, the cells of which are more 

 crowded toward the mesothelium, so that we may say that there are already im- 

 perfectly differentiated two layers of mesenchyma within the rib. The denser 

 layer next the mesothelium is destined to become still more marked and to trans- 

 form itself into the connective- tissue layer of the peritoneum. With the over- 

 lying mesothelium it develops into the peritoneal membrane of descriptive 

 anatomy. 



Sagittal Section. Through the Right Lung and Kidney (Fig. 145). The lungs 

 occupy a position in the upper part of the figure and are easily recognized by the 

 conspicuous entodermal bronchi, bro, which resemble in microscopic structure 

 the bronchi of earlier stages. The branches are widely separated from one 

 another by the voluminous mesenchyma of the organ. The lung is covered by 

 mesothelium, msth, and projects into the pleural cavity, Pleu. c, which is lined 

 by a continuation of the mesothelium of the lung itself. The pleural cavity can 



