W. K. BROOKS ON THE GENUS SALPA. 27 



from the double, transparent, vesicular nuclei of the follicle cells, as is 

 well shown in Fig. 8, where the blastomeres are marked bl and the follicle 

 cells 7 and S. 



Finally, the blastomeres begin to multiply actively by karyokinesis, 

 as shown at b in Fig. 9, and to give rise to the germ layers; but as it is 

 difficult to understand the peculiar relation between the follicle cells 

 and the blastomeres, without some knowledge of the history of both 

 structures, it will now be necessary to take up the history of the follicle 

 before we study the history of the germ layers. 



Stated in a word, the most remarkable peculiarity of the salpa 

 embryo is this. It is blocked out in follicle cells which form layers and 

 undergo foldings and 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 Tunicates show something similar, but 

 very much less pronounced. In another chapter I shall try to show how 

 the life-history of Salpa has come about, but we must now confine our- 

 selves to the facts. 



An imaginary illustration may help to make the subject clear. 

 Suppose that while carpenters are building a house out of wood, that 

 brickmakers 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 

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

 house of wood 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 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. 



