280 ME. G. BROOK ON THE 



have no record of the early segmentation at higher temperatures. 

 The difference is somewhat as follows : — Whereas at 49° F. it 

 took 2| days for an egg to advance from stage of fig. 9 to that 

 of fig. 12, at 60° F. this was accomplished in 22 hours. 



Sect. 2. From the first Formation oftJie Embryo to the Closure of 

 the JBlastopore. 

 The embryonic area encroaches considerably further on the 

 segmentation-cavity than is shown in fig. 10 before the first traces 

 of the embryo make their appearance. Shortly before this takes 

 place semidetached and quite detached cells make their appear- 

 ance about the apex of the embryonal shield. These cells rarely 

 have a rounded shape, but are mostly more or less angular, and 

 often have a pointed prolongation. These detached cells, so far 

 as I could make out, are lying loose on the floor of the segmen- 

 tation-cavity. This, however, is here so shallow that they 

 appear to fill up the whole area in an " optical section," and 

 cannot be made out clearly in this way. Can these be cells 

 pushed up from the intermediary layer to take part in the forma- 

 tion of the alimentary tract, as suggested by Kingsley is the 

 ultimate purpose of the intermediary layer ? or are they detached 

 cells from the invaginated hypoblast or lower layer cells ? As 

 the embryonic shield encroaches on the segmentation-cavity, the 

 latter becomes shallower as well, until after the embryo forms 

 and the blastoderm extends still farther it becomes, as at first, a 

 mere fissure of separation between the blastoderm and the inter- 

 mediary layer. Eyder ('Bull. U.S. Pish. Comm.' i. p. 147), quoting 

 from an earlier paper in ' Forest and Stream,' asserts that the seg- 

 mentation-cavity does not disappear at a very early stage of em- 

 bryonic life, as Balfour and others supposed, but that it " is filled 

 with fluid, and grows with the growth of the germinal disk, as 

 the latter becomes converted into the blastoderm, and does not 

 disappear until some time after the embryo has left the egg as a 

 young fish, after remaining as a space around the yelk-sac as long 

 as a vestige of the latter remains." My observations appear to 

 confirm the view taken by Eyder. An optical section made 

 after the blastoderm has already crossed the equator of the egg 

 is given in fig. 14, where it will be seen a double line runs forward 

 from the headend of the embryo to the thickened rim, enclosing 

 a cavity which Eyder says is filled with fluid. This double line, 

 with its enclosed cavity, follows the course of the blastodermic 



