36 TEMPOEARY OKGANS. 



remove from the blood certain deleterious matters, 

 and discharge them in the form of perspiration and 

 sebaceous oily matter. 



57. These organs for the most part inactive during- 

 embryonic life. But to the embryo, with the exception 

 of the liver, the above organs are comparatively useless. 

 They are being formed for work, but are as yet in- 

 capable of work. Nevertheless, the work that will in 

 the future be discharged by them must be performed 

 somehow. Food must be introduced though the 

 digestive organs cannot be used, oxygen must pass 

 into, and carbonic acid out of, the blood, though the 

 lungs cannot act ; and various deleterious excre- 

 mentitious materials must be got rid of, though the 

 glands being formed for this special purpose are 

 inoperative. 



58. Temporary organs. There is an arrangement 

 specially to fulfil these duties for the time. Various 

 temporary structures which are discarded at birth, 

 or are gradually destroyed and removed after birth, 

 perform, and in a most perfect manner, the highly 

 essential operations which are afterwards carried out 

 in a very different way. From the blood of the 

 mother is taken the nutrient matter from which the 

 blood found in the vessels of the growing embryo is 

 prepared, and into the maternal blood some of the 

 products of decay are discharged. It likewise gives- 

 oxygen to the embryo's nutrient fluid, and receives 

 from it carbonic acid, which is ultimately excreted 

 by the maternal respiratory apparatus. 



59. The Skeleton. The firm internal bony skeleton, 

 characteristic of all vertebrate animals, and which 

 supports all the other tissues of the body, is developed 

 at a time when no such support is required. When 

 the skeleton is first formed, it is as soft as the soft 

 tissues, and it only gradually acquires firmness as the 

 form of the several bones manifests itself. In the soft 

 state it is quite useless for the purposes for which 



