:. : /.:":/: : 



; ; ... 



animal, since it leads a subaquatic existence, being placed under water (Fig. 

 1) and deeply buried in the maternal tissues, which would account for the 

 peculiarities that obtain in its circulation approximating it to the stages in de- 

 velopment represented in fishes and amphibia. 



FIG. 1. GRAVID HUMAN UTERUS AND CONTENTS, showing the relations of the cord, placenta, 

 membranes, etc., about tbe end of the seventh month. 1, decidua vera ; 2, decidua reflexa ; 

 3, chorion; 4, amnion. (After Dalton.) 



The physical conditions under which the embryo is evolved determine 

 the special vascular arrangements for effecting circulation, while the trans- 

 formations which accompany this provide for the radical physical changes 

 which are ushered in at the end of the term, when it becomes an air- 

 breather, the fundamental circumstance underlying it all being an adjustment 

 ivith pressure, and the power of effecting rapid rhythmical changes in pressure, or 

 of producing the universal pumping actions going on in the body. For ex- 

 ample, we have seen that the rhythmical expansions and contractions pervad- 

 ing the body in the air-breather, and known as respiration, compel oxygen and 

 aliment in the circulatory apparatus for evolving force and producing growth ; so, 

 in like manner, a similar necessity exists in the embryo for compelling the nutri- 

 tive and force-producing elements in its circulation for producing growth and 

 evolving force, which is principally expended in elaborating its structures. 

 But since the embryo feeds in the uterine sinuses from which the com- 

 merce is obtained, and into which the waste products are returned, this calls 

 for the differentiation of the special organ known as the placenta, and which 

 answers to the more highly differentiated lungs and intestinal canal which are to 

 substitute it at the end of the intra-uterine term as the relative adjustments with 

 the larger environment, lower pressure, and higher order and amount of work 

 which this involves.* The placental souffle, then, which is distinctly heard 

 through the maternal structures is the analogue of respiration in the air- 

 breather, the relative ratio of the movements to the pulsations in the foetal heart 



For further particulars, see Part II. of this work, "On the Relation of Gravitation to 

 Development," to be issued shortly. 



