FIFTH PEBIOD OF DEVELOPMENT. 



Transition to the Larvae Stage. 



"We will include together in this fifth period of 

 development those stages which form the transition 

 from the development of the embryo, which takes 

 place at the expense of the yolk granules stored up 

 in the cells, to the larvae stages which are self- 

 nourishing. 



The processes which characterize this fifth period 

 of development consist in the perforation of a number 

 of apertures, which first render possible the functional 

 activity of the organs. These openings are : The 

 mouth, and the first gill-slit, the opening of the 

 ciliated organ, which has developed from the anterior 

 diverticulum of the alimentary canal, on the left side 

 the opening of the club-shaped gland, and finally, that 

 of the anus. These openings originate chronologically 

 in the order in which they have been here enumer- 

 ated. 



