232 



Sl^UDY OF PIG EMBRYOS. 



cells, the nuclei of which are themselves also somewhat flattened. This single 

 layer of cells is known as the epitrichium, because the hairs are developed en- 

 tirely underneath it. Where the epidermis is thickest, one can observe that the 

 layers of cells next to the mesoderm are closely packed together with round 

 nuclei. They represent the commencing formation of the basal layer of the adult 

 epidermis. Between the basal layer and the epitrichium the cells are more 



Vert. 



D.R. Ec.gl. Sp.c. an. G. 



Nch. 



Ve' 



Ve." 



card. 



N.io. (E. hro. I.u. PI. I a. 



Fig. 136. — Pig, 17.0 mm. Transverse Series 51, Section 464. 

 Ao% Aorta, bro, Entodermal bronchus, card. Posterior cardinal vein, cin. Neurone layer (cinerea) of spinal 

 cord. Cost, Anlage of ribs. D.R, Dorsal root. Ec.gl, Ectoglia. G, Ganglion. Li, Liver. Lu, 

 Lung, muse. Dorsal musculature. N.io, Vagus nerve. Nch, Notochord. (E, CEsophagus. PI. cue. 

 Pleural coelom. R.D, Ramus dorsalis. R. V, Ramus ventralis. R.sy, Ramus sympathicus. Sp.c, 

 Spinal cord. Sym, Sympathetic ganglion. Ve' , Ve" , Branches of the subclavian vein. Vert, Vertebra. 

 X 22 diams. 



loosely placed, forming the initial stage of the mucous layer. The mesenchyma 

 is very much developed and occupies a large territory in the dorsal region of the 

 embryo. It carries the nerves and blood-vessels and shows at various points 

 accumulations of more darkly stained cells, which are of two kinds : first, groups 

 of mesenchymal cells proper, the anlages of portions of the skeleton ; and, second, 



