124 Mr. W. K. Brooks on the 



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 tliey are 



to occupy they undergo rapid multiplication and growth, and 



build up the tissues of the body directly, while the scafltblding 



*'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 some- 

 thing similar but very much less pronounced. In the chapter 

 of my memoir " 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 building 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," &c., Naples Mittheil- 

 ungen, i., 1882, and " Embryonalentwicklung der Pt/rosoma,^^ 

 Zool. Jahrb. iv. and v., 1891) has discovered and minutely 

 described the migration of the follicle ; but he has failed to 

 trace the history of the blastomeres, and believes that these 

 degenerate and disappear, and that the embryo is built up of 

 follicle cells. I find that all the follicle cells are ultimately 

 used up as food, and that the true embryo is formed from 

 blastomeres after the analogy of the rest of the animal 

 kingdom. 



The Aggregated Salpce. — During their development the 

 aggregated Salpse undergo complicated changes of position, 

 which render the interpretation of sections very difficult, and 

 as both Salensky (Morph. Jahrb. 1877, iii.) and Seeliger 

 (Jena. Zeitschr. 1885) have totally failed to understand these' 



