ORIGIN AND DEVELOPMENT 49 



glands, lymphoid or adenoid tissue in various 

 organs, the lining epithelium of the heart and 

 blood and lymph vessels (endothelium) the 

 red corpuscles of the blood, and probably, but 

 not certainly, the white corpuscles of the 

 blood. 



22. As already pointed out, the portion of the 

 ectoderm that takes no part in the formation 

 of the embryo is known as the trophoblast. 

 This structure by and by comes into relation 

 with the maternal tissues in the uterus and 

 an important organ is formed, the placenta. 

 By means of this organ the blood of the mother 

 is brought into close relation with the blood of 

 the offspring. A thin membrane and layers 

 of cells intervene, and both respiratory and 

 nutritional changes are carried on. The foetus 

 breathes by the placenta, receiving oxygen 

 from the mother's blood and giving up to it 

 carbonic acid. The placenta also supplies 

 materials for the nourishment of the foetus, 

 and no doubt proteins, carbo-hydrates, fats, 

 saline matters, and water are thus supplied, 

 for the growth of the foetal tissues. At this 

 period the tissues of the foetus are so nourished 



