452 SOME GENERAL PROBLEMS OF DEVELOPMENT 



even fright before the birth of her child. Often these cases are 

 given with great exphcitiness and accepted as unquestionable 

 evidence, without realization that the conclusion is no more than 

 an " after-which-on-account-of -which " without scientific founda- 

 tion. While it is true that general conditions of nutrition in the 

 mother may affect the foetus and that certain poisons in her blood 

 may diffuse into the blood of the embryo, it seems wholly untrue 

 that her mental states can affect the offspring or that injury to a 



Fig. 236. — Planarians, eggs, and young. Modes of development in which the 

 young are encased in protective shells and provided with nutrient material 

 occur even in some of the lower animals. For example, in the fresh water 

 planarians (Class Tnrhellaria, Phylum Plalyhelminthes, cf. Fig. 117, p. 240), 

 eggs, consisting of tough capsules attached by a stalk and containing several 

 zygotes and many yolk cells, are laid in protected places and the young hatch 

 as miniature adults (cf. Fig. 220, p. 423). 



A, adult planarians feeding on bit of meat. B, egg capsules attached to lower side of a 

 stone and juvenile planarians escaping from a capsule. (Drawn by George T. Kline.) 



particular part of her body can produce a similar change in the body 

 of the foetus. The idea of prenatal influence as it is commonly held 

 is a myth when judged by scientific standards, but there are cer- 

 tain conditions in the parent that may affect the embryo during its 

 development within the uterus. 



To understand the kind of prenatal influence that may be 

 effective, it is necessary to understand the structural relationship 

 between the parent and the foetus {cf. Figs. 234 and 237). The 

 placenta by which the foetus is attached to the uterus consists of a 

 maternal portion, containing capillaries of the mother's circulatory 



