REPRODUCTION AND DEVELOPMENT 279 



periences of the mother; others attribute mental traits of the off- 

 spring to mental states of the mother during the intra-uterine life 

 of the oflFspring. It may be set down definitely here that there is 

 absolutely no direct connection between the nervous system of the 

 mother and that of the embryo; the relationship is similar to that 

 of parasite and host. Many of the superstitious and pseudo-scientific 

 ideas concerning prenatal influence are therefore purely imaginary 

 and have no basis in anatomical nor physiological facts; any school 

 of pseudo-science that includes prenatal influence as one of its doc- 

 trines is utterly worthless so far as that basis is worked into its 

 teachings. In order for a maternal factor, a food, a poison, or an 

 emotional state, to affect the development of the embryo it must 

 aflfect the chemistry of the mother's blood in such a manner as to 

 influence the transfer of materials through the membranes in the 

 placenta. It is unthinkable that a fright by a mouse, for instance, 

 could be transformed into a blood ingredient that would pass 

 through the semi-permeable membranes of the placenta, traverse 

 the long vein that conducts the blood from the placenta to the 

 embryo, and in the embryo be transformed into a pattern in the 

 skin that resembles a mouse. 



It is possible for certain types of bacteria to penetrate the mem- 

 branes of the placenta, enter the blood stream of the embryo and 

 infect it. Disease acquired in this way is not strictly speaking an 

 inherited disease; it is merely an intra-uterine infection not different 

 in principle from infections of adults. 



Embryonic Membranes. The embryos of birds and of mam- 

 mals are enclosed in a sac derived from the ectoderm and outer 

 layer of mesoderm during the early period of development. This 

 sac is the amnion and is filled with the amnionic fluid (Fig. 191). 

 During development, then, these embryos are immersed in a liquid, 

 the principal ingredient of which is water. At the ventral mid- 

 region the walls of the sac turn toward the embryo forming a tube, 

 the UMBILICAL CORD. The human umbilical cord may be several feet 



