342 Thos. 11. Montgomery. 



anterior to each the "semicircular lobe" (this equivalent to what I 

 have called the cerebral ridge), and found the semicircular grooves 

 become the chief part of the "brain" ; he also found that the "lateral 

 vesicles" compose a part of the "brain," and described the origin of 

 the eyes. Pappcnheim (1903) reached conclusions that are in close 

 agreement with my own ; from his fovea semicircularis of the head 

 lobe arises the cerebral ganglia, from the vesicula lateralis the optic 

 ganglion, and from the cheliceral ganglia the oesophageal commis- 

 sures ; he observed the abdomen to have independent ganglion pairs 

 for the first six segments, and three or four fused ganglionic pairs 

 behind those — the abdomen thus possessing more neuromeres than 

 somites. 



17. Blood Cells and Heart. 

 The blood cells first dift'erentiate when limb Ijuds begin to appear 

 on the abdominal segments, and they develop entirely from the extra- 

 embryonic blastoderm. This area is the one lateral and dorsal from 

 the embryonic body which consists of but one layer of cells, ectoblast, 

 and which previous to the proliferation of blood cells had furnished 

 those vitellocytes that are formed after gastrulation. At numerous 

 points, blood islands, of the extraembryonic ectoblast the cells in- 

 crease in size and the chromatin of their nuclei enters the cytoplasm 

 to compose coarse chromidia ; such cells then sink beneath the ecto- 

 blast to lie upon the yolk. These blood cells are the largest of all the 

 cells and have exclusive origin from the extraembryonic ectoblast iii 

 regions where there is no mesoblast. Subsequently they move along 

 the surface of the yolk into the embryonic region, where they come 

 to lie for the most part between the yolk and the embryonic body, 

 though some of them may penetrate into the mesoblast or even into 

 the coelom. At reversion the extraembryonic area becomes obliterated 

 by overgrowth of the body around the yolk, and then ceases the 

 formation of blood cells from the blastoderm. This reversion nar- 

 rows the extraembryonic area to a dorso-median space, the cavity of 

 the heart, bounded on either side l)y the dorsal ends of the somites; 

 these somite walls then overreach and underreach the blastocoelic 

 heart cavity and the wallof the heart tube is a product of splanchnic 

 mesoblast ; the coelomic cavity lateral of the heart tube is the begin- 



