CHAPTER XIV. 

 THE ONTOGENY OF THE ORGANS OF SALPA. 



SECTION 1. The Salpa Embryo. 



As I have already stated, the most remarkable peculiarity of the 

 salpa embryo is this : It is blocked out in follicle cells which form layers 

 and undergo other changes which result in an outline or model of all the 

 general features in the organization of the embryo. While this process 

 is going on the development of the blastomeres is retarded, so that they 

 are carried into their final positions in the embryo while still in a very 

 rudimentary condition. 



Finally, when they have reached the places which they are to 

 occupy, they undergo rapid multiplication and growth, and build up 

 the tissues of the body directly, while the scaffolding of follicle cells is 

 torn down and used up as food for the true embryonic cells. 



No other animal presents us with an embryonic history quite like 

 that of salpa, although other tunicata show something similar, but very 

 much less pronounced. In the chapter on the morphological significance 

 of the salpa embryo, I attempt to show how the life-history of salpa has 

 come about, but we must now confine ourselves to the facts. 



An imaginary illustration may help to make the subject clear. 

 Suppose that while carpenters are building a house of wood, brick- 

 makers pile clay on the boards as they are carried past, and shape the 

 lumps of clay into bricks as they find them scattered through the build- 

 ing where they have been carried with the boards. Now, as the house 

 approaches completion, imagine that bricklayers build a brick house 

 over the wooden framework, not from the bottom upwards, but here 

 and there, wherever the bricks are to be found, and that, as fast as 

 parts of the brick house are finished, the wooden one is torn down. To 

 make the analogy more complete, however, we must imagine that all 

 the structure which is removed is assimilated by the bricks, and is thus 

 turned into the substance of new bricks to carry on the construction. 



Salensky (Neue Untersuchungen, etc., Naples Mittheilungen I, 1882, 

 and Embryonalentwicklung der Pyrosoma, Zool. Jahrbucher, IV and V, 



