22 METAMORPHOSES OF MAN 



increases, and their quicker and more constant vibra- 

 tions disturb the body which carries them. This little 

 being, which is no longer an egg and is not yet an 

 animal, seems almost to poise itself upon the plate of 

 glass placed in the field of the microscope. At last it 

 undergoes its transformation. Suddenly the young 

 larva escapes, as if urged by some mechanical force, 

 and swims round and round, in the liquid, looking 

 just like a little hedgehog covered with sharp 

 spines. 



Hermella and Teredo are animals which undergo 

 two metamorphoses. We shall leave for a while these 

 eccentric larvae, but to study them at a later period, 

 and will return to the mammalian egg. 

 • We have seen that within the already large vitelline 

 membrane, the germ lay, surrounded by the blastoderm, 

 and that the germinal area was formed upon a portion 

 of this membrane. The germinal membrane almost 

 in its earliest stages is composed of two layers, which 

 may be exposed by a little delicate manipulation. 

 Very soon a third one is developed between the other 

 two, and grows with them. All the organs, and also 

 the various membranes which constitute the envelopes 

 of the germ, are developed from these three layers. 

 A special stratum formed from the external layer 

 becomes the amnion, which like a veil of gauze sur- 

 rounds the embryo, and secretes a large quantity of 

 liquid in which the young mammal is plunged up to 

 the moment of its birth. Another detaching itself 

 gradually from the same point, doubles within the 

 vitelline membrane, and helps to form the chorion, 

 which is a kind of egg-shell. From the two other 

 aminge there is formed, at the posterior end of the 



