The Development of Tlicridium. 329 



mesoblast and defiiiite entoblast. It is not nntil the stage of the early 

 abdominal lind) buds that the entoblast distinctly segregates from the 

 mesoblast in the abdominal region, and it is not a continuous layer ; 

 not until the stage of reversion does the entoblast form a tubular mid- 

 gut, then only within the caudal lobe and in consequence of the eleva- 

 tion of this lobe above the embryo, while even at the stage of reversion 

 the entoblast is still a discontinuous layer in the abdomen anterior to 

 the caudal lobe. The definite entoblast arises only abdominal, none 

 of it originates with the cephalothorax, for the whole gut (stomo- 

 daeum) of the cephalothorax is ectoblastic; it segregates relatively 

 late and even at the time of reversion lines the yolk only posteriorly 

 and ventrally. The cephalothorax possesses only ectoblast and meso- 

 blast. The vitellocytes do not form entoblast, and though my study 

 does not include stages late enough for me to decide this point, the 

 evidence is that the vitellocytes take no part in the formation of the 

 intestine, but play much the same role as the periplast (periblast) 

 cells of vertebrates. After the gastrulation new vitellocytes are 

 formed from the extraembryonic blastoderm. 



Claparede (1862) described a central ''cumulus primitif," which 

 is at first a rounded eminence, then elongates to pyriform shape, 

 extending back to the "pole anal" and proliferating cells along its 

 whole length ; at the stage of the early segments he identified it with 

 an eminence placed dorsally between head and tail lobes. Then Salen- 

 sky (1871, Theridium) found a funnel-like invagination developing 

 in the middle of the germ disc, the opening of which finally closes, 

 and the cells proliferated from it wander beneath the whole blasto- 

 derm; before this invagination closes there arises behind it a transi- 

 tory mound of blastoderm cells. Balbiani (1873) corroborated 

 Claparede in the appearance of a primitive cumulus near the center 

 of the germ disc on the more flattened surface of the egg; at that 

 point the blastoderm is more than one cell layer deep, and it 

 is the center of that thickened portion that gradually elevates 

 itself; near it arises by cell proliferation a wider area, the 

 "tache posterieure," distinct from the cumulus ; the "tache pos- 

 terieure" represents the cephalic lobe, while the cumulus takes 

 a dorsal position (dorso-caudad from the cephalic lobe). Balfour 



