O H. E. JORDAN 



ALLANTOIC DUCT 



The allantoic duct (fig. 1, All.) is shown highly magnified in 

 figure 4. It has an irregularly oval shape in cross-section. Its 

 wall is thrown into deep folds. Within the depth of the folds 

 the epithelium consists of a single layer of cuboidal or even 

 squamous cells, comparable to its condition throughout in the 

 21-mm. fetus; over the crests the epithelium is of the stratified 

 columnar type, consisting of from three to four layers of cells. 

 No distinct basement membrane can be discerned other than as 

 indicated by the deeper red color of the immediately subjacent 

 connective tissue after picric-acid-fuchsin counterstain. The 

 nuclei have an irregularly oval form; their wall is wrinkled, and 

 the nucleoplasm generally lacks a distinct network or granules. 

 A number of the cells are hollowed out centrally. The cells next 

 the lumen have a more condensed, probably slightly keratized, 

 broad distal border. The enveloping connective tissue is less 

 differentiated and denser than in any other portion of the section. 

 It resembles early embryonic connective tissue or young mesen- 

 chyma. It contains many vasofactive cells ('angioblasts'), and 

 one large blood-island (B. I.), which will be described below. 



THE CONNECTIVE TISSUE 



The character of the connective tissue varies in different 

 portions of the transverse section (compare figs. 2,3,4, and 5). 

 In the region immediately surrounding the allantoic duct (fig. 1) 

 it is least differentiated. Here it is compact and resembles 

 embryonic connective tissue or young mesenchyma (fig. 4). In 

 this region also are numerous vasofactive cells (fig. 15). Along 

 the peripheral border of the specimen the connective tissue is 

 somewhat more differentiated and represents an older type of 

 mesenchyma (fig. 2). Here the cells have generally a stellate 

 or fusiform shape and are more widely separated. In this region 

 also the capillaries are most abundant; these for the most part 

 lie along the radii of the section and appear to be growing towards 

 the ectodermal covering. Here also are found abundantly initial 

 stages in the formation of the blood channels (figs. 8, 9, and 10). 



