330 Tlios. H. Montgomery. 



(1880, Agelena) was the first to employ actual sections in the study 

 of these relations ; he found a cumulus appearing near the edge of the 

 germ disc ("ventral plate"), near the center a larger "white area," 

 and determined that a line connecting the two marks the future long 

 axis; he is inclined to identify the white area with the future pro- 

 cephalic lobe, and the cumulus with the caudal lobe. Cell invagination 

 takes place at least in the region of the white area, and the cumulus 

 is "the point where the first traces of the future mesoblast become 

 visible." The thickening of these areas is due mainly to cell multipli- 

 cation, but in part also to addition of cells from the yolk. The ven- 

 tral plate then divides into epiblast and mesoblast; the splanchnic 

 mesoblast becomes formed by yolk cells adding themselves to the ven- 

 tral plate, and so probably arises the intestine (that appears much 

 later). According to Balfour, accordingly, both splanchnic mesoblast 

 and entoblast are formed from yolk cells adding themselves to the 

 embryo. Locy (1886) found a depression succeeded by a primitive 

 cumulus "at one end of the flattened surface of the egg * * * a 

 second thickening, which I shall call the caudal thickening, now 

 makes its appearance on the flattened surface of the egg, at a distance 

 of about 80° from the cumulus." It increases rapidly in size, spread- 

 ing out most in the direction of the cumulus, and ultimately becomes 

 shield-shaped. In the region between these two structures the ventral 

 plate is gradually formed by a blastodermic thickening, which is 

 not at first continuous with the two terminal thickenings. Cell 

 proliferation occurs in the region of both eminences. In the stage 

 of the protozonites the embryonic area consists of ectoderm and meso- 

 derm, but Locy has not presented detailed observations on the segrega- 

 tion of the two mesoblast layers and first appearance of the entoblast. 

 Lendl (1886) observed on the blastodenn a "Primitivhiigel" that 

 comes to take a caudal position, and a "weisser Fleck" ; the yolk 

 cells compose the entoblast. Morin (1887) found that from the 

 center of the embryonic thickening cells separate themselves, some 

 to enter into the yolk as entoblast cells and others to remain upon the 

 yolk as mesoblast. The cumulus of the authors is present in Pholcus 

 and Drassus, but absent in Theridion, and arises after the germ layers 

 are well formed ; it consists of a mass of mesoblast cells, later it sepa- 



