328 BIOLOGY. [BOOK iv. 



vestment has been called serous or animal vestment, because it 

 is the rudiment of most of the apparatus and organs of the 

 animal life. The internal vestment is called mucous or vegetative 

 vestment. From it proceed the nutritive apparatus. 



Finally the cells accumulate at a point of the blastoderm, and 

 form there a sort of spot called the embryonic spot. It is there 

 that the embryon develops itself. 



In their ensemble the phenomena we have just described are 

 common to all vertebrated and invertebrated animals, endowed 

 with sexuate ovulation. But, starting from the formation of 

 the embryon, differences arise corresponding first of all to the 

 two great branches of the animal kingdom. To follow step by 

 step these differences would be evidently to go beyond the limits 

 assigned to us. "We must content ourselves with indicating 

 some general facts. 



The division of the animal kingdom into the vertebrated and 

 invertebrated branches is of all the most general, the most im- 

 portant. Also it is marked at the very outset of the embryonary 

 x ,e volution. In the vertebrated ovulum the embryonary spot 

 elongates into an ellipse, rises in the middle after the fashion of 

 a buckler, and at this point the serous vestment hollows out for 

 itself a rectilinear furrow (primitive line, nota primitiva). The 

 edges of the furrow rise more and more, and in the depth is formed 

 a solid cellular cord, a sort of provisional scaffolding, round which 

 the vertebral column is to be formed. This cord is the notocord or 

 dorsal cord (Figs. 51 and 52). 



It is to special treatises that the reader must go to follow the 

 ulterior phenomena of development, the formation of the um- 

 bilical vesicle, of which the content serves still for the nutrition 

 of the young being, that of the amnios, which envelopes it in 

 an aqueous medium, the origin of the placenta or of the vascular 

 organ, which, in the superior viviparous vertebrates, establishes 

 a communication between the circulatory system of the mother 

 and that of the embryon, when this last has exhausted the 

 nutritive resources of the vitellus. 



