680 A NEW ENDOPAKASITIC COPEPOD, 



the gastrula, stain a lighter colour with borax carmine, than do the 

 remaining cells. Even in the sixteen-celled embryo, in some cases, 

 the posterior quad has a lighter tinge than the other twelve, and 

 almost invariably a certain number of the posterior cells of the 

 sixty-four celled embryo take tliis lighter tinge. From these cells 

 are derived the whole of the internal organs; they constitute an 

 endo-mesoderm rudiment. 



The embryo as we left it in the last section, therefore, consisted 

 of a wall of ectodermal cells, at the blastopore, exposing cells of the 

 endo-mesoderm rudiment, the main body of which form a solid core 

 to the embryo. 



Whether tlie ectoderm grows over, and closes the blastopore, or 

 whether the endo-mesoderm remains exjDosed here even in the 

 nauplius, I was unable to decide. Some sections appear to evi- 

 dence one condition and others the other, so that 1 leave the matter 

 misettled, but with a decided inclination to assert that the former 

 condition exists, i.e., the blastopore is closed. 



The cells of the solid endo-mesoderm core soon increase in size, 

 more especially those around the central axis. 



Meanwhile, the ectodei*m cells of the anterior pole become so 

 differentiated, that a cap of them take a^ darker stain from haema- 

 toxylin than the rest. 



There next follows a fatty degeneration of certain of the more 

 centrally-placed endo-mesoderm cells, resulting in a mass of fat 

 globules. Fig. 41 represents the embryo just before the fatty 

 degeneration has properly set in. Along with the fatty degenera- 

 tion of some of the cells of the endo-mesoderm core, there is growth 

 posterior-wards of the anterior cap of differentiated ectoderm, to- 

 getlier witli a bilaterally symmetrical disposition of the margin of 

 tlie same ; of this symmetrical disposition, the most obvious feature 

 is a division on the ventral side in the midline, and a pair of capes, 

 to use a geographical term, wliich project beyond the general out- 

 line on either side of tlie median bay. 



A rearrangement, and differentiation of the endo-mesoderm cells 

 next takes place, accompanied by an intensification of the bilater- 

 ality of the ectoderm, and the first appearance of the appendages, 

 on either side of the "bay." (See Fig. 42.) 



