98 EXPERIMENTAL STUDIES ON FETAL ABSORPTION. 



transfer of products from the embryotrophe to the fetal circulation. Furthermore, 

 it is richly vascularized and consequently the amniotic fluid is completely surrounded 

 by fetal blood-vessels which would make it difficult to conceive of dye-stuffs diffus- 

 ing through the membrane without first entering the fetal circulation. Moreover, 

 from the present observations and those on the guinea-pig recorded in the study just 

 preceding, it appears probable that dyes such as trypan blue, when administered 

 in large doses, stain the fetus as well as the amniotic fluid. The portal of entry of 

 these traces of dye could be either the placenta or the vitelline membrane, or both, 

 but it appears likely that, whichever route the dye pursues, it first enters the fetal 

 blood-stream. 



In the human being it is theoretically conceivable that fluid may diffuse 

 directly from the maternal blood-stream into the amniotic fluid. The wall of the 

 amniotic cavity contains no fetal vessels and the chorion is intimately fused with 

 the uterine mucosa in which prominent maternal vessels are visible. If fluid 

 escapes from these vessels it would seem quite possible that some of it might diffuse 

 through the amniotic membrane directly into the amniotic sac. 



It is of interest to consider the differences which exist in various animals in 

 regard to the staining of the amniotic fluid after injection of a vital dye into the 

 maternal blood-stream. Goldmann (1909) first noted the staining of the amniotic 

 fluid in the rat and mouse after the administration of vital dyes. The author has 

 observed the passage of traces of trypan blue into the amniotic fluid of the guinea- 

 pig and rabbit. In similar experiments on cats (Wislocki, 1920), however, the 

 amniotic fluid, as well as the allantoic fluid, remained unstained. The conclusion 

 to be drawn from these observations is that the fetal membranes and placentae of 

 rodents are slightly permeable to ultra-microscopic particles, such as trypan blue, 

 while those of carnivorous animals are impermeable. 



Homer (1905) made parallel observations upon the permeability of the mem 

 branes and placenta in different classes of animals. He injected tetanus antitoxin 

 into pregnant sheep, cows, rabbits, guinea-pigs, and human beings. He observed 

 that the antitoxin passed readily from mother to fetus in the human, that it was 

 transmitted only occasionally in the guinea-pig and rabbit, and that transmission 

 never occurred in sheep or cows. What is the explanation of this striking difference 

 in behavior? Romer concluded from his experiments that the more heterogeneous 

 the substance the more readily is it transmitted by the placenta. Thus tetanus 

 antitoxin, which was derived from the horse, was extremely heterogeneous for man 

 but only slightly so for the cow and sheep. Another explanation suggests itself, 

 both for Romer's results and for the passage of traces of trypan blue through some 

 placentse and not through others. We must recall that Grosser (1909) has classified 

 placenta3 according to the degree of union which exists between the fetal and maternal 

 tissues. Thus, in the pig, cow, sheep, etc., the simplest types, there occurs merely 

 an apposition of the chorionic ectoderm to the unbroken surface of the uterus. In 

 carnivora, such as the cat and dog, which represent the next highest type, the 

 chorionic ectoderm has invaded the uterine mucosa so that the placenta is made up 

 of maternal blood-vessels intimately surrounded by fetal tissue. In rodents and 



