NOURISHING MEMBRANES 



495 



with members of the group that very clearly nourish the young with a 

 fluid food furnished by the walls of the uterus. 



We shall study, as an example of this latter histological condition, 

 the uterine wall of the elasmobranch fish Myliobatis aquila, a ray found 

 in the Mediterranean Sea and stud- 

 ied by Brinkmann. In the uterus 

 of this fish the lining epithelium 

 is simple, and is arranged on a 

 series of papillae to present a large 

 underlying blood supply to the 

 surface. This is just as it was 

 arranged in Acanthias. 



In addition to this arrangement, 

 however, such of the epithelium as 

 lies in the spaces between the 

 papillae is specialized so as to be 

 able to take materials from the 

 blood, and, after having elabo- 

 rated them into a food material 

 that is particularly adapted to 

 the needs of the embryo, to dis- 

 charge them into the uterine fluid 

 in which the embryo lies, where 

 they will be accessible for its nour- 

 ishment. 



This food material is discernible 

 in the uterine fluid as a milky con- 



sec. g 



FlG. 464. Myliobatis aquila; a higher mag- 

 nification of one of the nutritive pockets 

 shown in the last figure, sec.g., cytoplasmic 

 secretion granules; lu., leucocytes. (After 

 BRINKMANN.) 



stituent, and while its chemistry and the physiology of its assimilation 

 by the embryo have been studied, there remains much interesting work 

 to be done. Figure 463 shows a low power, outline view of a vertical 

 section of this uterine lining to show the relations between the respiratory 

 and nourishing epithelium; while in Figure 464 is shown a much enlarged 

 view of a section of one of the "pockets" of nourishing epithelium. It 

 can be seen here that cellular elements (leucocytes), as well as the se- 

 creted materials, are passing through or between the epithelial cells. 

 The secreted substance is observable as a series of vacuoles which are 

 slightly larger in the distal cytoplasm than in the proximal. The writers 

 believe, from study of Brinkmann's figure and without having seen 

 actual sections of this material, that the two kinds of epithelial nuclei 

 described by this author are the same, of which the lighter ones are such 

 as have been cut by the knife in sectioning, and thus have stained more 

 slightly. 



We shall next study the extreme of this condition, as found in a 



