50 EXPERIMENTAL STUDIES ON FETAL ABSORPTION. 



cent solution of iron ammonium citrate and potassium ferrocyanide was injected into 

 the amniotic sacs of several fetuses. The membranes were fixed in acid formalin 

 at intervals varying from 5 minutes to an hour. Evidence of the diffusion of the 

 injected fluid was gained by finding Prussian-blue precipitate in the membranes. 

 Very soon after the injection this precipitate was found in the amniotic epithelial 

 cells, as well as between them. It also appeared, after a longer interval, in the 

 subepithelial mesoderm. This demonstrates the gradual penetration of the amni- 

 otic membrane by the injected solution. 



To summarize these observations, it may be stated that during the latter part 

 of pregnancy there are three pathways of absorption from the amniotic sac. The 

 chief of these appears to be direct transudation through the amniotic membrane. 

 The other two routes are through the fetal stomach and respiratory tract, 



After noting the passage of vital stains and true solutions through the amniotic 

 membrane, it was of interest to determine the further course which they pursue in 

 entering the fetal circulation. This course is necessarily determined for each 

 animal species by the relationship of the amniotic membrane to the chorion, allan- 

 tois, and yolk-sac. The guinea-pig is very similar in development to the mouse, 

 which has been carefully described by Sobotta. The striking feature in both is that 

 amnion and embryo develop within the blastodermic vesicle instead of upon its 

 surface, as in other mammalia. Furthermore, the yolk-sac surrounds the internal 

 cell-mass and amnion; subsequently, the wall of the blastodermic vesicle and the 

 outer layer of the yolk-sac atrophy, leaving the fetus with a layer of endodermal 

 cells — the inverted yolk-sac — as its outermost covering. This endodermal mem- 

 brane persists throughout fetal life, becomes richly vascularized by the omphalo- 

 mesenteric vessels, and develops numerous villi whose cells are markedly phagocytic 

 and probably play a large part in the absorption of the embryotrophe and in the 

 nutrition of the fetus (Goldmann) . Due to the obliteration of the extra-embryonic 

 ccelom, the amnion, early in fetal life, becomes loosely adherent to this membrane 

 by strands of mesoderm. As a result, the amnion is brought into intimate contact 

 with the omphalo-mesenteric vessel (figs. 1 and 2). It can be demonstrated that 

 these blood-vessels are partly responsible for the absorption of solutions from the 

 amnion. To do tins, a solution of iron ammonium citrate and potassium ferrocy- 

 anide is injected into the amniotic sac. One hour later the membranes of the fetus 

 are fixed in acid formalin and the omphalo-mesenteric vessels are examined under 

 the microscope. Precipitated Prussian blue is encountered outside as well as inside 



the vessels (fig. 6) . 



AYe now pass to a description of the fetus in the second phase of vital staining, 

 namely, the appearance of the vital dye in the form of granules in groups of cells, 

 collectively called macrophages. As we have described, trypan-blue passes from the 

 amniotic cavity into the circulation of the fetus. After 24 to 72 hours it appears 

 in the form of characteristic granules in many of the fetal tissues. It must be 

 pointed out that this vital staining is not comparable to that obtained after repeated 

 injection in the adult. As is well known, the several tissues of the adult animal do 

 not stain equally rapidly or heavily, and after a single injection one may fail 



