23 



The tubules include every variation between those with narrow 

 lumiua and columnar or polyhedral cells, and those with very wide 

 lumina and very much flattened epithelium. They vary in length from 

 100 to 200 microns. 



The appearance of the tubules is such as might be expected to 

 result in consequence of pressure exerted by a fluid content. That 

 such was probably the case is indicated by the fact that the surrounding 

 mesenchyme is crowded aside and the cells very much flattened. But 

 for some internal pressure forcing the tubules to enlarge and the cells 

 in consequence to flatten, the confining mesenchyme would undoubtedly 

 have held the tubules in the earlier cylindric form. In Fig. 4 is 

 shown a cross section of characteristic tubules. Fig. 7 shows portions 



Fig. 7. Camera lucida drawing of transverse sections of portions of three contiguous 

 entodermal tubules from the older vesicle (13 mm embryo), showing the character of 

 the lining cells. In A the cells are irregularly polyhedral, with a vacuolated coarsely 

 fibrillar cytoplasm. Staining capacity varies greatly. The smaller cells generally contain 

 a denser, more nearly homogeneous and deeper staining cytoplasm. Some of the cells 

 (7") contain large deeply staining masses; others (U) contain numerous irregular gra- 

 nules; both types of inclusions are more probably cell detritus. In tubule B the ento- 

 dermal cells have become greatly flattened. The cell broders are disappearing. The 

 nuclei are becoming smaller and homogeneous and stain faintly. The cytoplasm is 

 finely granular and stains deeply in the portion to the left. Tubule C represents the 

 extreme type. Here all cell outlines are lost and the nuclei are small and pale {W). 

 The cytoplasm stains more deeply. The several tubules are separated from each other 

 by connective tissue; the line of demarkation between entoderm and mesenchyme is 

 distinct. The tubules are filled with an amorphous coagulum identical with that in the 

 cavity of the vesicle. The vesicle is lined in the regions where the epithelium is 

 stratified by the type of cell represented in tubule A. In the regions where the epi- 

 thelium is simple the cells are of the type shown to the left in A or more generally 

 of the type shown above in B. The difference between the entodermal cells in the 

 younger and older vesicles seems to be one of greater degeneration in the latter. Original 

 magnification, 1000 diam. Reduced ^/^ in reproduction. 



