PREGNANCY 



69 



The foetus contains parts of the original three embryonic 

 layers — ectoderm, mesoderm, and endoderm— and from each of 

 these layers certain particular organs are produced. 



The ectoderm gives rise to the epidermis and its a])pendages, 



Fig. 51. — Human embryo, about eight and a half weeks old. 

 (After His, from Grmjs Anatomy.) 



the epithelial structures of the sense organs, the epithelium 

 of the nasal passages, mouth, and distal part of the urethra— 

 the enamel of the teeth, the epithelium of the skin glands and the 

 glands opening into the vestibule of the mouth and nasal passages, 

 the' muscle fibres of the sweat glands and those of the iris, and 

 the whole of the nervous system. 



The mesoderm gives rise to the skeletal muscles (except 

 those just mentioned as arising from the ectoderm), the various 

 connective tissues, the heart, the blood and lymph vessels, the 



