AND THE LOWEK ANIMALS. 11 



In all vivipara, on the contrary, the vitellus is very- 

 small. It would not suffice for the nutrition of the 

 embryo, which is obliged to draw from external 

 sources the materials requisite for its further de- 

 velopment. 



On account of this difference alone, it follows that 

 certain germs can separate themselves completely from 

 the parent, and others are forced to remain for some 

 time in the interior of the latter. The egg of ovipara 

 with a large vitellus is laid, that is to say expelled 

 from the body, and often abandoned to every external 

 influence without any protection save a delicate mem- 

 brane or a thin shell of a mineral character. The egg 

 of vivipara, left in a condition of complete activity, en- 

 grafts itself upon the maternal womb like a parasitic 

 plant, absorbs from it the nutritious juices which it 

 gives to the embryo, and grows pari passu with the 

 latter. The phenomena which it exhibits, called into 

 action by the necessity which exists for the nutrition 

 of the young animal, alter its nature in no way, and at 

 the final moment its special features make their ap- 

 pearance. In coming into the world, the mammal 

 and man burst through thin envelopes, just in the 

 same manner as a bird breaks through its shell. 

 Birth is actually a process of hatching. 



Now, in certain species the embryo, once fully formed 

 and arrived at the foetal condition, resembles its 

 parents. At the moment of emergence from the 

 shell, it presents almost the general form that it will 

 maintain during life. The mode of performance of the 

 principal functions is definitively fixed, and though a few 

 organs may still be imperfectly developed, yet all are 

 present, and none disappear. The alterations which 



