AND THE LOWER ANIMALS. 21 



area. It is tlie place where the creative forces exert 

 their greatest power, or rather it is the embryo itself. 

 The area increases very rapidly, and becomes oval. 

 A transparent line appears in the direction of its 

 greater axis. This is the primitive streak which 

 already indicates the position of the future brain and 

 spinal cord — the two great nervous centres that con- 

 trol the entire organism. Next, a series of obscure 

 points placed symmetrically along this line shows us 

 that the vertebral column is being formed. 



The sub -kingdom being thus fixed on, the class to 

 which the animal will belong is afterwards deter- 

 mined. 



In Mammalia, as in the others, the proper coat — 

 vitelline membrane — undergoes change also. Al- 

 though at first thick and uncovered, it is soon sur- 

 rounded by a layer of albumen, which is blended 

 and grows with it. The egg is still free and un- 

 attached ; but it soon throws out a number of delicate 

 folds, [the first rudiments of the series of rootlets 

 which at a later period it will dip into the substance 

 of the mother's womb,] for the purpose of absorbing 

 the juices intended for the nutrition of both the 

 embryo and itself. 



In Hermella and Teredo, as soon as the blastoderm 

 is formed, the vitelline membrane, hitherto inactive, 

 commences its labours. Its irregular folds disappear, 

 it increases in thickness, and, like a sort of flexible 

 egg-shell, it is fitted exactly to the incomplete embryo, . 

 as though it were an actual epidermis or skin. A few 

 cilia are seen on its surface. At first they are as- 

 motionless as crystal filaments, but afterwards they 

 commence jerking violently. Their number rapidly 



