The Development of Theridium. 331 



rates from the mesoblast to occupy a dorsal position and give rise to 

 blood cells. Bruce (1887) described the primitive cumulus as well 

 formed before the blastoderm is complete. "It is very probably 

 formed both by the division of cells which have reached the surface 

 and by the addition to these of yolk cells" ; from it comes the meso- 

 blast. Kishinouye (1890) stated that near the center of the blasto- 

 derm there arises a '^primary thickening" that is to be compared 

 with a blastopore, and a little later a ''secondary thickening" ; the 

 former in widening pushes the latter to the margin of the germ disc 

 (but whether the latter lies anteriorly or posteriorly is not stated). 

 The germ layers form from both thickenings, but more specially from 

 the primary. ''Cells -^ - * proliferate into the yolk and be- 

 come scattered without any definite arrangement through the entire 

 yolk. These are the entoderm cells." Then Schimkewitsch (1887, 

 1898, 1906) has given the most detailed study of all. x\ll the cleavage 

 cells reach the surface to constitute the blastoderm, then some of them 

 (yolk cells or vitellophags) sink secondarily into the yolk. A cumulus 

 primitivus arises at the middle of the blastoderm, and then a "zweiter 

 Fleck" before (sic!) this ; these become later connected. The cumulus 

 is the point of formation of the mesentoblast and also of additional 

 yolk cells, along the line of a leng-thened blastopore. In the anterior 

 region of the embryo the mesoblast and entoblast become early dis- 

 tinct from each other, while at the posterior end (caudal lobe) em- 

 bryonic mesentoblast continues to proliferate mesoblast and entoblast. 

 From these and from the vitellocytes Schimkewitsch distinguishes 

 mesoblastic phagocytes, cells that arise wherever mesoblast cells touch 

 the yolk ; they are smaller than the vitellocytes, but, like these, ingest 

 yolk. The dorsal cumulus of Claparede and Morin is held by 

 Schimkewitsch to be a true tubular dorsal organ and not a part of 

 the blastopore. The entoblast consists of the posterior anlage, from 

 which arise the cloacal sac, the malpighian vessels and the intestinal 

 epithelia; and of scattered cells on the yolk surface that form the 

 epithelia of the liver sacs. 



It will be seen, then, that there is much confusion with regard to 

 the formation of the germ layers. The consensus of opinion, op- 

 posed only by Balfour and Bruce, is that the yolk cells sink from 



