22 



the blood islands and vessels, and is invaded by entodermal tubules 

 (frequently jutting into the capillaries) and cords of entodermal cells. 

 These will be described in detail below. The amount of connective tissue 

 in diiferent regions of the wall varies inversely as the number of 

 entodermal tubules. 



The Mesothelium. 

 The mesothelium has become eroded over a great part of the 

 surface. Where present it appears healthy and consists of cubic or 

 flattened cells with oval nuclei. Ciliated borders as described by Branca 

 could not be seen. 



The Tubules or "Glands". 



Every portion of the wall contains the cylindric or flask-shaped 

 entodermal tubules ("glands", Spee ; "crypts", Selenka). They appear 

 slightly less numerous in this vesicle than in that of the 9,2 mm 

 embryo. They are evidently beginning to disappear. According to 

 Meyer they seem to have disappeared in umbilical vesicles of embryos 

 over 60 days old. Also, their lining epithelium is less regularly colum- 

 nar or cubic than in the younger embryo. In neither vesicle was 

 ever any mesenchyme found in the tubules as reported by Meyer (33). 

 All contain an amorphous coagulum identical with that in the cavity 

 of the vesicle. 



The tubules can usually be traced into the main cavity where 

 their epithelium becomes continuous with the entoderm lining the ve- 

 sicle. Frequently they open by very constricted necks, sometimes 

 almost occluded. Total closure and subsequent separation result in a 

 certain number and form blind tubules. These become cystic. Their 

 number increases in the older vesicles. 



Immediately beyond the neck the tubules bend almost at right 

 angles and pass distally or sometimes proximally parallel to the long 

 axis of the vesicle. Sometimes a second limb passes in the opposite 

 direction, the whole arrangement forming with the neck a Y- or T- 

 shaped structure. Tubules have been seen to branch once but never 

 more. In a cross section of the sac the great majority of the tubules 

 are cut transversely showing their regular longitudinal arrangement. 

 The tubules are in both vesicles very much more numerous distally 

 and on one half of the surface. A number of them lie very close to 

 the entodermal lining (Fig. 4). In some instances the "tubules" are 

 solid cords of cells, the result more probably of a failure of original 

 separation than of subsequent proliferation. 



