160 Mr. T. T. Groom. [June 16, 



After separation of the epiblast the yolk cell or macromere remains 

 still as a cell with a single nucleus, derived from that of the merocyte 

 which formed the last or one of the last blastomeres. This yolk cell 

 represents both mesoblast and hypoblast. 



The meso-hypoblast cell immediately divides into two cells, one 

 situated more dorsally, the other more ventrally. Each of these 

 contains mesoblastic and hypoblastic elements. The mesoblast is 

 formed by the cutting off in succession of segments from each of the 

 two meso-hypoblast cells ; these form a plug of rapidly dividing cells 

 just in front of the closed bla^topore. When all the mesoblastic cells 

 are cut off the two yolk cells left remain as the first two hypoblast 

 cells. 



The two cells thus formed become divided into smaller yolk 

 endoderm cells equivalent to the secondary yolk pyramids of Decapods 

 (Reichenbach's Secunddre Dotterpyramiden) . 



Each yolk pyramid becomes later converted into an endoderm cell 

 by radial contraction in a centrifugal direction, accompanied by 

 gradual retreat of the nucleus to the periphery; the archenteric 

 cavity arises by the separation thus caused of the central portions of 

 the pyramids from one another. 



The alimentary canal arises in three divisions, as in the Arthropoda 

 generally, the stomach being forme^. mainly from the yolk endoderm, 

 and the lining of the oesophagus and intestine probably wholly as 

 long epiblastic ingrowths (stomodaeum and proctodoeum). 



The mesoblastic cells of the nauplius, arising in the way described, 

 divide up rapidly and extend forward between the ectoderm and 

 endoderm as a dorsal plate ; this soon grows down at the sides, but 

 does not at first extend to the ventral surface. 



This plate is chiefly concerned in the formation of the muscles of 

 the nauplius appendages, which arise, as is probably the case in all 

 nauplius forms, with the free ends directed dorsally. 



The appendages are marked out first by two transverse furrows 

 dividing the embryo into three segments ; these occur only across the 

 dorsal surface and up the sides, not extending into the ventral 

 surface. Very soon the dorsal surface becomes traversed by a median 

 longitudinal furrow, which does not extend to the ends of the body, 

 but is bounded by two new transverse furrows ; these furrows 

 mark out an anterior and posterior (caudal) unpaired lobe with the 

 free ends of the appendages between them on the dorsal, and not, as 

 has always been stated for Cirripedes, on the ventral surface. 



The autennules, antennae, and mandibles are probably serially 

 homologous, as indicated by their similarity in the free nauplius, and 

 by their remarkable and similar origin ; all may represent primitively 

 post-oral appendages. 



There are no mesoblastic somites at any period of embryonic 



