370 CEPHALOCHORDATA. 



sacs, lying along the inner wall of the atrial cavity in 

 segments 10-36 on each side. In reality they lie in little 

 compartments of the coelome. 



In the mature female the ovaries are large and con- 

 spicuous ; the ova burst into the atrial cavity, whence they 

 pass into the pharynx and out by the mouth, or more 

 directly by the atrial pore. 



The testes are like the ovaries; the spermatozoa burst 

 into the atrial cavity, and pass out by the atrial pore. It is 

 likely that the eggs are fertilised in the water. 



Development. — The fertilised ovum segments completely 

 and equally; the resulting blastosphere is invaginated to 

 form a gastrula. 



Along the mid-dorsal line of the gastrula, the epiblast 

 cells form a medullary groove, which becomes a medullary 

 canal, and thus the central nervous system is formed. 



The cavity of the gastrula — the archenteron — becomes 

 the gut of the adult. 



The notochord arises along the mid-dorsal line of the 

 archenteron. 



From the archenteron a coelome pouch grows out on 

 each side. Each is divided from before backwards into a 

 segmental series of sacs. The cavities of these sacs form 

 the body-cavity ; their walls the mesoblast. 



There are several anterior pouches. Two grow forward, 

 one on each side of the notochord. These form the 

 most anterior part of the body-cavity. But there is also 

 an unpaired sac which extends forward, and divides into 

 two. Its right half seems to have no special history, but 

 the left forms the organ of smell or taste which we spoke of 

 as opening on the roof of the mouth. 



The larva is ciliated during the early part of its life, and 

 is much more active than the adult. 



