92 Majnard M. Metcalf on the 



showing that in the later stages the follicle-cells composing 

 the outlines of the organs are replaced by true blastomeres, 

 which give rise to the adult. He says (page 27) : — " 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 tinal positions in tlie embryo while still 

 in a very rudimentary condition. Finally, when they have 

 reached the places they are to occupy tiiey 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." 



Brooks's tigs. 1 and 2, plate xlii., also ^^^. 12, pi. ix., as 

 well as his descriptions, demonstrate that the peculiar granular 

 bodies seen within the blastomeres at certain stages of deve- 

 lopment are not an indication of the fragmentation or degene- 

 ration of the blastomeres, but are nuclei of follicle-cells that 

 have been ingested and are undergoing digestion. This 

 statement I have fully confirmed, as described a few pages 

 beyond. The amitotic division of the migrating follicle-cells 

 confirms the belief that they are on the road to degeneration, 

 and in the centre of the embryo there are I'ound masses of 

 such disintegrating cells. 



lieider's account of the embryology of Salpa fusiformtSj 

 published in 1895, diflfers in certain points from preceding 

 accounts. 



He interprets the granular bodies in the protoplasm of the 

 blastomeres as ingested follicle-ce//^, and ligures them as 

 containing nuclei (pi. i. figs. 4, 10 a, 10 6), and largely from 

 this observation argues, as Brooks had shown, that the follicle- 

 cells serve as food for the blastomeres. 



Heider places emjjhasis upon the unequal cleavage of the 

 Sa/paoYXiw, claiming, contrary to Salenskyand Brooks, that, 

 except in the early stages, the micromeres cannot be distin- 

 guished from the follicle-cells, and that organ rudiments, 

 which are apparently formed from follicle-cells, are really 

 composed of small blastomeres. 



The insulhcient reference in Heider's paper to Brooks's 

 monograph may perhaps be exj)laincd by the laet that lieider's 

 paper was practically complete before Brooks's work was 

 published. 



KorotnefF's several papers arc the most recent dealing with 

 this subject. This author denies lieider's contention that 



