THE ILGG 



processes (villi) all about it. As it grows it bulges into the 

 uterine cavity, remaining rooted at its base, and thus forms 

 a definite organ of attachment, the placenta. The sketch, 

 Fig. 14, gives a diagrammatic idea of this arrangement, and 

 Plate XII illustrates some of the details of implantation in 

 man and monkey, from the unique specimens of the Carnegie 

 Embryological Collection. 



Within the placenta, blood vessels of the embryo ramify 

 in close proximity to the blood stream of the mother. Nutri- 

 tive substances and oxygen filter from the uterus through 

 these blood vessels into the infant's blood; waste substances 

 and carbon dioxide filter out. The infant, completely im- 

 mersed in the fluid contents of its dark chamber, thus gets 

 only such substances as can be brought to it dissolved in the 

 mother's blood and filtered through the placenta. 



Preparation of the uterus for implantation. It may make 

 the next chapters clearer if we anticipate slightly at this 

 point our discussion of the corpus luteum hormone (Chapter 

 V). While the embryo is floating into the uterus prior to its 

 attachment, it requires nourishment from the uterus. To 

 provide this, and also to favor the invasion of the maternal 

 tissues by the placenta, the hormone made by the corpus 

 luteum (progesterone), acts on the uterus, causing great 

 activity and growth of its tubular glands. Without this 

 preparation the embryo, arriving in the uterus, would be 

 unable to develop, like the seed told of in the Biblical parable, 

 that fell on stony ground. 



{ '59 } 



