PROBLEMS OF MAMMALIAN DEVELOPMENT 



453 



system, and a foetal portion, containing capillaries of the embry- 

 onic circulation. In general, there seems to be no direct con- 

 nection between these two blood systems, since the introduction 

 of injection fluids into the capillaries shows their complete inde- 

 pendence. Anything that passes between parent and offspring 

 must, therefore, be in the form of a diffusible substance, and 



-ID 



-2 



Fig. 237. — Diagram of a section through the human placenta at the middle of 

 the fifth month of pregnancy. The maternal blood sj'stem and tissues are 

 intimately associated with those of the f(i>tus, although the two circulations 

 are distinct and material is interchanged between parent and offspring only 

 by diffusion. The technical designations of parts by the author of the figure, 

 as given below, are not explained, but the student can interpret the essential 

 facts to be illustrated by understanding that the area with horizontal lines 

 above represents tissue of the embryo; the black, maternal blood spaces; 

 and the parts below the black other maternal tissue. 



.4, amnion; bl, afferent niaterna! arteries; BP, basal plate or decidua basalis; Ch, chorion; 

 Ff, f, and h, chorionic vascular villi of enxbryo; i, intervillous blood spaces or lacuna) (mater- 

 nal blood); m, chorion; ^f, nuisculature of the uterine wall; r, marginal sinus; up, spongy 

 layer of decidua basalis; Tr-tr, line along which the placenta will separate at parturition; 

 Z, Chorionic villi; *, closing plate. (After Hertwig, from Kellicott, "Chordate Develop- 

 ment," copyright, 1913, by Henry Holt and Co., reprinted by permission.) 



experiments show that not all diffusiljle substances can pass with 

 equal readiness between the two systems. The embryo of a mam- 

 mal is a separate organism developing within the mother in a 

 relationship which resembles that of a parasite to its host. At no 

 stage is there any relationship whatsoever between the nervous 

 system of the fcstus and that of the mother. The idea that her 

 mental states can be impressed upon the offspring necessitates the 

 absurd belief that nerve impulses can pass through the placenta, 



